I applied through university. The process took 4 months. I interviewed at Penn State (Reading, PA) in Aug 2011
Interview
If you perform well in class, you may be asked to join the teacher's assistant program. You will then undergo a semester long class discussing the pertinent issues involved in aiding in the classroom. As long as your grades do not decrease and their are no major issues with your work ethic you will be a teacher's assistant until graduation.
Interview questions [1]
Question 1
Dealing with students who are smart, but have no work ethic or motivation.