Question: Based on the ending, when Veronica invites Marsha to hang out, I have the impression that she doesn't intend to confess her involvement in the fake "suicides." People will continue to think that Heather was depressed, and Kurt and Ram were secretly gay. Is there some reason, other than jail, for her to let everyone keep believing the lies?
Answer: People who were bullied by Heather, Kurt, and Ram might feel better because of the faked suicides: they believe that the bullies were actually struggling with issues. As one boy says, he now thinks that Heather quit dating him because she was dissatisfied with her own life, not because he wasn't good enough. In the locker rooms, a girl tells Veronica that she thought Heather was just a bitch, but "guess I was wrong."
Question: Why did JD expect anyone to believe his plan for the bomb at school - that all of the students signed a mass suicide note? Nobody would think that so many people, with different personalities and interests, agreed to die together. He later tells Veronica, "Pretend I did blow up the school, all the schools" - implying that he wanted to do this again. He might be crazy, but he was careful in how he executed the murders of Heather, Kurt, and Ram. Why the ridiculous bomb plan?
Answer: He doesn't care if anyone believes it or not; he is trying to make a statement about society and that no matter how we see ourselves in life, we are all the same when we're dead.
Answer: Not only is JD crazy, he is overconfident now. He and Veronica got away with the other murders. He thinks a fake mass suicide could be successful.
Answer: The students didn't know they were signing a "mass suicide" pact. They thought it was a petition to win a free concert from a popular band. JD covered the suicide wording on the paper with a sticker that made it look like a contest entry. He pulls off the sticker and reads the real message after he finds Veronica hanging in her room.
That was not the question. It's a question of why JD thinks anyone will believe the note, after all the students are dead. Has nothing to do with the students knowing what they were signing.
Question: After Veronica convinces Heather McNamara not to kill herself, she suggests they "knock off early". Are they just going to skip class, or are they possibly allowed to leave school if they are seniors? At least, during the time when this movie was made (before the increased security concerns of recent years).
Answer: Yes, Veronica meant skipping class. And, as seniors, they may have been allowed to leave the school grounds.
Answer: She means to leave school early; whether they are allowed to or not cannot be determined from the context of their conversation.
Answer: I think it's one of those "This can't be happening, I must be dreaming" moments. Same as when a character pinches themselves in order to wake up, or they tell someone to pinch them. Veronica can hardly believe what she and JD did.