This girl at school has a crush on me, but she is the ex of one of my homies and I really like her what should I do

Dating her could pose as a potential problem with your friend even if he won't admit to it, it will still bother him in some way.
It's disrespectful in all view points if you think about it, because girls in general, have little respect for those who betray a friendship...
And, you never know... She could be using you to try to get back at him for some reason... and what better way than to destroy one of his friendships...
I would leave her be and only date girls that have not already dated your friends.

Well homie, id say ditch the chicks and worry about school!

But put yourself in your homies shoes and ask yourself if you would care if he went after an ex of yours!

Why don't you ask your homie how he would feel if you asked her put?
Honesty is the best therapy.