I think it was the whole idea of using violence. Keep in mind
-The school blows up
-All of the students carry weapons.
Oh, I hated writing that down. It was an overreaction. The students had to use weapons, and the school had to go. If they cancelled graduation, the mayor would ascend somewhere else, and there would not be 100 people to at least try, plus slayer would be a no-go .I guess it was just out of fear.
And although this reason may be completely minor, it technically happened at night (remember the 3:28pm sun blacking out), which changes the whole entire feel of the school. It was (I think?) on a Saturday, not a reg school day at 11:00am. If you keep in mind the context, there is nothing wrong with the BTVS arc.