After starting the "Halloween" mission, you can get a bonus for finding and smashing all the pumpkins and tombstones around campus. If you do not find all of them, do not worry. In the next (and last) mission of the chapter, where you go into the basement of the school with Gary, you will get to the boiler room. Inside will be all of the pumpkins you missed in the "Halloween" mission. Kick all of them for an unlockable. Later in the game when you unlock the "Preppie" part of town, go to the asylum. Next to it will be a warehouse, and inside are all of the tombstones you missed.