Psycho? thriller with teens

Henlo,

this is a movie I watched as a kid, must be around 1999 to 2005. The movie itself looked like it was made in the same years. I cant really remember the general story, but I think there were two men in their teens involved in a murder. I remember that one of them was especially intelligent and/or somewhat strange/timid. In the end the crime was solved because the other one couldnt stand their secret anymore and told the police, but I’m not sure on that one. I can only recall one short scene in which the timid guy had an appointment with a girl to help her on her college stuff. I think it was math related but could have been anything. Before they meet I recall her watching through her window (1st floor, single family house?) and see him arrive, not sure about that one either though ^^ Set in the US, looked east-coastish.