Can't Hardly Wait

In 1998, the senior class of suburban high school Huntington Hillside High are attending a graduation party at the house of a rich class member. Among them is Preston Meyers, an outsider who plans to proclaim his secret four-year-long love to Amanda Beckett.
Amanda, the most popular girl in school and the senior class prom queen, has been dumped by popular jock Mike Dexter. His classmate William Lichter targets him, plotting revenge against Mike for years of bullying. Kenny Fisher is a wannabe thug who plans on losing his virginity by the end of the night.
Amanda is consoled by her popular girlfriends, but realizes she has nothing in common with them. She tries to figure out if she has an identity beyond only being known as "Mike Dexter's girlfriend".
Amanda discovers a letter addressed to her by Preston and, moved by its contents, makes it her mission to find him, though she does not know what he looks like and no one she asks gives any helpful descriptions. Meanwhile, Preston's antisocial best friend Denise Fleming and Kenny accidentally lock themselves inside a bathroom away from the party, where they talk about their old friendship and how they had drifted apart. Their conversation leads to the restoration of their friendship and escalates into them having sex.
A graduate from Mike's high school, Trip McNeely, tells him that guys like them are "a dime a dozen" in college. Trip emphasizes how he dumped his girlfriend in the same fashion that Mike did to "score" with other women and was unsuccessful. Terrified of this prospect, Mike tries to get Amanda back, but she is happier without him and humiliates him in front of everyone there.
After seeing the school jock turned down, multiple guys begin to hit on Amanda, much to her disgust. Preston finds her and professes his love, but as she still has not learned what he looks like, she assumes he is another pervert and rejects him in front of the entire party. Amanda later realizes her mistake when she sees Preston's yearbook picture and tries to find him, but he has already driven home in disappointment.
Meanwhile, William devises his plan to get revenge on Mike and goes into the party to drive him out. While there, William begins drinking alcohol to fit in, drinking enough to make him forget why he was there. An impromptu sing-along to Guns N' Roses' "Paradise City" causes him to become popular, with multiple women trying to have sex with him.
William later begins talking with Mike, who apologizes for bullying him. After William forgives him, the two bond and seemingly become friends. When both are jailed as a result of a police bust, Mike takes the blame.
The next morning, when William sees Mike and his friends at a diner, he tries to thank him for taking the fall. However, Mike acts as though he remembers nothing that happened the previous night, and again ridicules him in front of his friends.
Meanwhile, Preston is at a train station about to leave for Boston when Amanda arrives and asks him about the letter. He confesses he wrote it and is about to depart for a writing workshop with Kurt Vonnegut. The two say goodbye and Preston walks away, but then stops and runs back to Amanda before the two kiss.
The epilogue explains what happens to all of the main characters:

William becomes one of the most popular students at Harvard. He forms a computer company that makes him worth millions, and starts dating a supermodel.
Mike goes to college but, after drinking too much, loses his football scholarship. He ends up forty pounds overweight and working at a car wash, a job he loses when incriminating Polaroids surface.
The day after the party, Denise and Kenny go to a diner, where she dumps him five minutes later. Ten minutes pass before they find a bathroom and get back together.
Seven hours later, Preston boards a train to Boston. Amanda writes him a letter for every day that he is away. They still remain together.
In a mid-credits scene, William's two friends are beamed up by a flying saucer.