Slaughter High

The day begins with popular student Carol Manning (Caroline Munro) jokingly asking school nerd Marty Rantzen (Simon Scuddamore) to have sex with her in the women's locker room. After he removes his clothes, Marty is surprisingly exposed to a group of students who tease him in several ways, including jabbing at his crotch with a javelin, giving him an electric shock, and dunking his head into a toilet bowl. This prank ends when the students's coach comes in to find out what is happening.
The students are forced to perform a vigorous workout in the gymnasium. Still, two students give Marty a marijuana joint laced with poison, which he smokes in the science lab where he is working on a chemistry project. The joint makes him so sick he runs to the men's room to vomit. While he is gone, one of the popular kids named Skip rigs the experiment to explode in Marty's face. Upon Marty's return, the set up works, causing Marty to panic and a great fire to erupt in the lab after he knocks over a Bunsen burner. In the struggle, Marty accidentally bumps into a shelf where a precariously placed jug of acid sits and causing the jug to fall and break, splashing acid on Marty's face and leaving him horribly disfigured. The sound attracts the attention of the students, who stand by the door in shock.
Years later, those behind the prank are mysteriously invited back to the school for a reunion. Upon arriving, they discover that the campus was closed years ago and the building is in disrepair and awaiting demolition. They begin to think that one of the former students was behind the invitations, but everyone denies it. They decide to break into the school to hang out and drink alcohol, then come across a room containing their old school lockers which, to their surprise, are filled with their old belongings. They notice Marty's old locker, and the alumni begin questioning what happened to him after the prank gone awry. Skip tells them that Marty has supposedly been kept in a mental institution.
Soon, the caretaker is killed onto a coat peg by a jester masked assailant. Soon thereafter, the friends begin dying in gruesome ways - Ted's stomach is melted from acid, Carl is impaled in the back inside a car, Shirley melts away in an acid bath, Susan arrives late and is attacked, Stella and Frank are electrocuted on a bed while having sex, and Joe is slaughtered by tractor blades. After all the deaths, Carol, Nancy and Skip, soon find that the bodies have disappeared. Skip is later hanged in a noose by the killer. Carol and Nancy investigate and find a room playing a video of Marty being bullied by them. Nancy runs off by herself and is chased by the Jester, where she falls into a cesspit and drowns. Carol is left alone and discovers Susan's body before being chased by the Jester inside the old school. In the process, Carol mistakenly kills a survived Skip with a hatchet. After a long run, Carol is ultimately impaled on a javelin pole, and the Jester is revealed to be Marty himself. After this, Marty sees the victims coming back as revenants and they attack him. He then awakes screaming, inside a mental hospital while restrained to a bed and wearing bandages on his face. While being checked on, Marty proceeds to strangle a nurse and dresses in her uniform before killing a doctor with a syringe in the eye. Vowing to actually get revenge, Marty turns to look into the camera and begins to tear off half of his face, before the screen fades to black.

Question: WHY WHERE THEY INVITED BACK TO THE SCHOOL?
Answer: REUNION