Go ahead and skip this if you don't want to know anything about what happens in School Days. None of this is really important; it's just musings about the definitive "nice boat" ending of an anime series.
Now:
In the last episode of SD, Sekai kills Makoto, then gets killed by Kotonoha, who went nuts in the previous episode. (Well, who was shown to have gone nuts in the previous episode, anyway--doing things like "talking to Makoto" via a dead cell phone, etc.) And at the end, Kotonoha is sailing into the sunrise aboard her family's "nice boat", with Makoto's head cradled in her bosom.
So I was thinking about all this, and I came up with the following timeline.
8 AM: Makoto tells Sekai to go see his friend at the hospital about an abortion.
3 PM: At Makoto's place, Sekai tells Makoto she didn't go. Sekai sends him a text message ("Makoto [a shitload of carriage returns] ...sorry") and then stabs him to death. She leaves, taking the murder weapon with her, but leaving Makoto dead on the floor. (Kotonoha was either there or arrived right after Sekai left. In any case she knows that Sekai killed Makoto.)
8 PM: Sekai gets a phone message from "Makoto" asking to meet her atop the school building.
9 PM: Sekai arrives atop the school building with the knife she used to kill Makoto. Kotonoha is there, with Makoto's head in a duffel bag. The girls fight and Kotonoha kills Sekai and then checks her abdomen for a baby, and finds none.
Morning: Kotonoha sails into the sunrise, "nice boat", etc.
This is what we are shown in the episode (with approximate times). Here's what I think goes on "behind the scenes".
10 PM: After a long day at work, Makoto's mother arrives at home. Seeing Makoto laying on the floor, she chides him for not sleeping in his room...until she sees the pool of blood and the stump of a neck. She screams and runs, and the to-do is enough to ensure the police are summoned.
The police quickly determine that the head was severed some time after death. The coroner can give a quick-and-dirty estimate of about 3 PM for "time of death", but that's really a no-brainer since that would be after school anyway. The police ask about Makoto's friends. It's hard to say what Makoto's parents know about his friends and love life, but it's safe to say that the police would ask about a cell phone, and it would be quickly ascertained that Makoto's cell phone is not there. A trace order goes out.
Meanwhile, the parents mention one or two of Makoto's friends that they know, and the police contact them. The police would then learn about Makoto's bed-hopping and Sekai's pregnancy, so it would be natural for them to contact Sekai.
12 AM: Sekai's parents report that she's not home, and they don't know where she went.
The next morning, it would be natural for the police to go to the school and interview teachers, students, etc. In the case of a brutal murder/mutilation I expect they'd be there first thing in the morning, in fact, and in short order they would learn about Kotonoha, and a list of students who did not come to school would be generated and run down. The absence of Kotonoha--who is somewhere at sea at this point--would generate curiosity and the police would question her parents, to learn that she is not at home, either.
The police might be waiting at the school for the "person on duty" to open the front gates. Or perhaps they would arrive there just as someone--going to the roof for one reason or another--finds Sekai's body and calls the police.
Next to Sekai's body the police would find the knife she used to kill Makoto, though they don't know this yet. They find the puddle of vomit (Sekai's) nearby (near the place that Kotonoha had set the duffel with Makoto's head.) They'd find her cell phone, and soon they would know that a message from Makoto's cell phone--sent hours after his death--had brought her here. That would make them wonder who had sent it; certainly Makoto had not. It's not certain whether or not they would find the "Makoto...sorry" message in her phone's out box. There's no way to tell for sure, since it really depends on what Sekai set her phone to do.
The wound in the abdomen, opines the coroner, was made after Sekai's death.
But if the police search Sekai's room, they may find bloodstained clothing. I recall that she had gotten Makoto's blood on her when she killed him. (Common sense would suggest she had not worn bloody clothing to go back to the school, but who knows? She managed to get home while wearing it.)
From this point, then, it only takes a little while for someone to wonder if perhaps Kotonoha went to the marina where her family's yacht is kept; and it would then rapidly develop that the yacht was not there.
Right now the police only know that someone has killed both Makoto and Sekai, and that Kotonoha is missing--she's a "person of interest", not a suspect. They want to find her and question her; and so whatever equivalent Japan has to the US Coast Guard moves into gear to find that yacht.
By the time the search begins it has to be mid-afternoon.
Assume that Kotonoha isn't going to kill herself; she's loopy enough that she's happily cradling Makoto's head on the deck of the boat. The boat has an autopilot (it would have to in order to sail as it's shown sailing) but the autopilot can only do so much, so it's not going to sail at the boat's top speed. Figure an average speed of maybe three knots; in fifteen hours (assuming Kotonoha left right after killing Sekai) the boat would be 45 miles from harbor. That's an approximate semicircular search locus, with a fifty-mile radius, centered on whatever marina the boat started from. A couple of search boats with good radar sets and a couple of decent planes should be able to cover that area pretty rapidly.
Assume, then, that Kotonoha is found before dark and "rescued". The searchers find the Head of Makoto in her possession; also Makoto's cell phone. She's arrested.
Then it comes out pretty quickly that Sekai killed Makoto; that Kotonoha severed Makoto's head and took it with her, summoned Sekai to the roof of the school building, and killed Sekai, and then did a casual check of Sekai's uterus for a baby. (Considering that the baby would be smaller than a golf ball, I am not convinced that Kotonoha's dissection of Sekai's uterus would be enough for a thorough inspection. Particularly not in subdued light.)
After the forensic work is done, the police uncover more facts:
1) the blood on Sekai's clothing is not all hers. Some is Makoto's.
2) the blood on the knife by Sekai's body is Makoto's.
3) the "Makoto...sorry" message in Makoto's phone establishes Makoto's time of death. It comes from Sekai's phone and--together with the blood evidence--proves that Sekai killed Makoto.
4) The message on Sekai's phone, sent from Makoto's phone by Kotonoha, was sent hours after Makoto died.
5) The autopsy of Sekai's body shows that she was pregnant; DNA tests show that the child is Makoto's.
6) Kotonoha is fruitier than a nutcake. A psychologist is brought in to determine if she's competent to stand trial for murder.
The police can then piece together the events of the previous day with some certitude, and in the end (IMHO) Kotonoha goes to jail for murder.
I think the way Kotonoha set up the meeting with Sekai shows premeditation. Her craziness might mitigate her sentence, but she's clearly sane enough to plan a murder and execute it such that the body is not found for hours, nor were there any witnesses. She might have escaped capture if she'd not insisted on taking Makoto's head sailing with her, but instead hopped a plane for someplace. (No one who saw her "talking to Makoto" knew she was talking to a dead phone, so that's not likely to come up in her defense.) Taking Makoto's head sailing would show that she was nuts, but I don't know if it shows her being nuts enough to be "not guilty by reason of insanity". People who are sane enough to stand trial can nonetheless do some pretty fucking horrible things for the most bizarre of reasons.
* * *
In the end, as I said, none of this is important or momentous in any way. It's just me thinking about the details of how things would go outside of the plot elements presented in the story.
I didn't think about the long-term repercussions of these events--what would happen to the characters' families, what would happen to the people at the school, and so forth. I mainly thought about whether or not Kotonoha would be arrested, or if the details of the deaths of Makoto and Sekai would just be filed under "WTF happened?"
One could make a case that Kotonoha ended up committing suicide, jumping into the (ice-freaking cold western Pacific in January) ocean with Makoto's head and drowning, leaving the "nice boat" to fend for itself--it might or might not be found, but Kotonoha's body and Makoto's head probably would not be. In which case the details of the "night of horror" would never be known, and probably the police would be looking for the mystery killer who killed Makoto and Sekai, and who must've kidnapped Kotonoha and killed her before disappearing...?
I guess I really want there to be some justice in this story, because at least in my version of the denouement Kotonoha has to answer for her crimes. (Both Sekai and Makoto end up being punished for theirs, certainly.)
* * *
Of course, there won't be a follow-on series to School Days. No sequel, unless it's Women in Prison Days or something. Heh.
BTW, why didn't anyone tell me I was misspelling "sequel" all this time? I've been spelling it "sequal" which is fricking wrong. Damn it.