It is good to forget the past but if it is painful. Good memories of the past are like flowers. They always smell nice & make you fresh & alive. If you try to forget knowingly they will come to your mind with force. Please relax & pray to Allah after every prayer for patience. You never know that after school you may get those friends back with you in collage & university.
If we just think that everything which happens is in the hands of Allah then we can agree to it. This is behaviour of a true Muslim who always agrees to the command of ALLAH.