how likely am I to get Covid if I take the vaccine this week and teach a class of 50 students (in a closed classroom) over the summer?

So far, the vaccines (all 3 - pfizer, moderna, J&J)have proven 100% effective in preventing death and even severe hospitalization. That doesn't mean you won't get COVID, but it does mean that even if you do the chances of you having any major difficulties is almost zero. Sort of like getting a cold or the flu, it is possible to get COVID (although still not that likely) but you probably won't be terribly ill if you do.

That being said, locked in a room with 50 students for 4 hours sounds like trouble. Definitely break that up and go outside have hour or so.

If you get the first shot this week, you'll get the second mid-May, and be fully vaccinated by the beginning of June. That makes it very unlikely that you will catch COVID. The students will still be wearing masks, right? I've been teaching this semester in a classroom- I got the second shot two days before I started teaching, so it wasn't fully effective yet, but neither I nor my students have had any problems or gotten sick.