Criminal Minds Crossover
Aaron Hotchner had to help raise his two year old brother, Connor, when his Father died. Sean was 12 at the time and would help out too. As the years passed and Connor grew into a stunning child and a beautiful young man Aaron and Sean became overprotective and wanted their brother safe and protected. Connor attends a boarding school and specialises in Languages but also takes Martial Arts. When a case brings the BAU to Connor's school because boys are disappearing and turning up dead Aaron finds out some new facts about his brother.
Possible that Connor is friends with some other supernatural people but the school is not a Slayer school. Also if Dawn is in the story then the pairing cannot be DawnConnor.
Connor must be better at fighting than the instructor and regularly kicks the instructors butt- but Aaron and Sean don't know this.