For M&M there I'd say the two most important books (other than the Core rulebook) are Freedom City and Ultimate Power.
Freedom City contains a full campaign setting and has lots of premade characters. Alternatively you can buy Hero High if you're planning on running things more like the X-Men...
Is it possible to have art, business studies, English, history and science, but no foreign language? I didn't think it was common to learn a second language in the U.S.
I think ATT would be best simply because most have accounts there. I'm not that fond of joining too many new boards, since I already feel that I'm on many that are taking attention to keep up with.