How do i tell a girl i can't be with her?

ok im leaving in a few months to go away to a boarding school cause i really hate my school and i wanna play good hockey. A really niceand cute girl i have had feelinngs for really wanted to go out with me and i explained to her that i would be leaving and that it would only crush her when i left but she really wants to be with me for a few months bt i really dont want to see her hurt cuase i know that shes the kind of person that can get crushed. i dont and have always kinda liked her but i really dont want enything to get in the way of my future. plus my mom is already upset about me wanting to leave at 15 but its what i want to do to improve my acedemics as wel as hockey. i just dont know what to tell her. we have always been pretty close and she told me that she cant stay away from me and that theres something bout me that she gets attached too . please help