There ia girl at my school and i really care about her, pls high school or middle?

i know this girl at my school and i really care about her. (i actually know her, not just one of those "love at first sight' crap.) but she is kinda throwin her life away. she is goin out with a a****** and she is changin in a bad way. grades, personality, looks, other stuf like that. she also picked up crack. terrible. but i want to help her. the only way to help her is to tell someone but i dont really want to rat her out like that. i really care about her but im torn!HELP