When a popular school clique viciously beats a boy to death because a two-faced classmate cried rape, 20 years later, a horribly disfigured 7ft. butcher traps them at a fake high school reunion to exact raw vengeance with a billhook.

I miss a main character, and an inciting event (in the present). The 'butcher' feels not organic, not specific to THIS?story.

You start the logline off with the word when, and go on to describe action that occurs 20 years later. This causes confusion as it's not WHEN?the inciting incident happens, rather AFTER?it happens that the story takes place. This error exposes a flaw in the concept, the inciting incident happens too long in the past for it to be significant enough for the main character. In other words, the real inciting incident is not a boy getting beaten up, it's?the appearance of a monstrous killer. This means the first sentence in your logline can be cut altogether, as it describes back story instead of plot.

Second big problem is the lack of a goal. It is implied that there will be a fight to survive, but surly the main character will need to not just survive the ordeal, but also stop the killer from harming others. They would need to either kill the killer or have them arrested, therefore best to describe this in the logline.

Lastly if the main character beat up another boy, I can't see the audience developing empathy for them. This raises the question; why would the audience care what happens to him at all?

You have not given us a lead character. Also you spend to much time in your logline on backstory.