I think it’s a movie but it might be a TV episode from a crime show, though I remember it as being quite long. It took place at a boys boarding school (in England maybe?) and a girls mutilated body was found in a greenhouse on the grounds and her intestines were hanging out or something. There were two boys sharing a dorm room and one of them had a bloody knife in the drawer and it was implied that it was him who was the murderer, and I think he might have had a creepy lab full of knives and dead animals and stuff in the basement under his parents house and he took the other boy there and showed it to him (they crawled in through the basement window) and in the end they were on a train and in some weird plot twist it was discovered that it was really the other boy who was the murderer and I think he might have killed the first boy by throwing him off the train? Sorry I can’t remember anything else, help would be much appreciated! Also btw it was in English.