Step 1: Email School.
Send them an Email saying something like this:
hi my name is _____ I use/currently are going to ______ school. I'm interested in computers, and I'm interested in (how computers work, programming, anything with computers should work.) I was wondering if you had any spare computer you would be willing to let go to a student for educational purposes.
Just something around that, you want to make it seem like you know what your talking about, that your interested in computers, one thing schools liek to is that its for educational purposes, and you want to seem professional.