The Van

Bobby and his best friend, Jack, sabotage their high-school graduation ceremony by rigging the stage to collapse. After leaving the graduation ceremony, Bobby and Jack (Harry Moses) drive up to another van and notice Sally (Connie Lisa Marie), his secret high-school crush, sitting alone inside, eating ice cream. Bobby cannot look away from her and expresses his fantasy to "be with" her just once. Dugan returns to the van and disapproves of Bobby's attention. Bobby and Jack run across Sue (Marcie Barkin) and Tina (Deborah White), and they attempt to arrange dates with them that evening. Sue is interested in Jack, but Tina states she is neither interested in Bobby nor other boys because they only care about sex.Bobby works in a car wash. Bobby's boss Andy (Danny DeVito) is also a bookmaker, who refuses taking a high-risk bet from an old lady called Bertha. Bobby's co-workers send his convertible into the car wash with the roof down, but Bobby plans buying a van anyway - not just any van, but a custom van with items like a waterbed, a ceiling mirror, a refrigerator, a toaster, an 8 track and multiple television sets. Bobby then gets revenge on his co-workers while showing off his new van by giving them beers spiked with laxatives.In a bar, Bobby unsuccessfully approaches women, including Sally. Finally, a pinball playing girl agrees sharing a joint in his van. Though going hand-in-hand, she objects his moves. Laughing it off, he tries raping her, but Bobby discovers she has stuffed her over-sized bra with wads of toilet paper, and she runs off. Up next is a Mexican girl, who removes her top but demands payment. Another woman is also a prostitute, and her pimp barges in for payment.Afterwards, Bobby and Jack force Sue and Tina to stop their car. Sue drives off with Jack, and Tina joins Bobby in the van. They plan to meet up again later at the beach, but Jack and Sue don't arrive until the next morning. Bobby and Tina form an uneasy truce and sleep in the van without fooling around. Eventually, they share a joint. When Jack and Sue return in the morning, Bobby makes them think that he is in the van having sex with Tina, who is actually out on the beach.After dropping Tina home, Bobby spots Sally with a see-through wetlook in the car wash. After seeing her making out with Dugan, Bobby stops by a quiet take-out place. The take-out waitress praises his van, so he offers sharing a joint on the waterbed. Noting she likes waterbeds, she says she doesn't have time for a joint, but does have some time for a quickie. She steps out from behind the counter and Bobby gasps at her very large buttocks. Once she is nude in the van, Bobby tries a pickup line, but barely finishes it, when she throws and mounts him on the waterbed. He protests, but she obviously rapes him, breaking the waterbed in the process. Bobby mentions her buttocks' size to Jack, and notes he never knew sex could be so physical.After tricking Bobby and Tina to join them to a van spot, at a local beach, Jack and Sue have sex in the back with occasional voyeurs in their window. Bobby and Tina see other vans, before Tina is nearly run over during the van drag races. Bobby and Tina get close, until Bobby again tries getting physical. Tina drives off in the van with Jack and Sue in the back, leaving Bobby to walk home.When Andy reveals he got beat up for not paying for a winning high-risk bet, Bobby volunteers his van's down payment. After getting rejected when approaching Tina's window, Bobby visits Sally's house. Sally says she feels abandoned by Dugan, and Bobby finds himself with her in his van. Unzipping him, she notes he is bigger than Dugan. Taking off her top, they proceed to have sex.The following morning, Tina runs across Bobby in a diner and agrees going together inside, where they meet Jack and Sue. Dugan then enters, lifts Bobby up and says he saw Bobby's van last night around Sally's place. Jack stalls Dugan, but Tina doesn't want to see Bobby again. Refusing to give up again, Bobby kidnaps her. Dugan drive chases Bobby, and the police join in. Bobby escapes, but Tina pleads to get freed. Bobby finally stops when she agrees hearing his side. He says he got the van to succeed with girls, but it changed upon meeting Tina. Tina says she too could never play "the game", and they have sex.The next morning, Bobby drag-races Dugan to regain his money, but not before mentioning he really was at Sally's place. Dugan tries crashing Bobby, but eventually crashes himself into a police car. In result, Bobby's van turns over. Tina, Sue, Jack and Andy are relieved when Bobby is neither dead nor injured. Jack notes Bobby even won the race (when the water seeping from the broken waterbed inside the van leaks out and across the finish line).Bobby leaves in Tina's convertible. Tina asks if he really was with Sally, and Bobby replies that Sally said he is "bigger than Dugan," but Tina just laughs.

Question: How do Bobby and his best friend Jack sabotage the graduation ceremony ?