Translation:Which school is he going to pick?
Also, 所 is the correct measure word for 学校, I think. It isn't a big deal, but duolingo is particular with other measure words, so I'd like it to be consistent.
I'm native speaker from Taiwan.
In my opinion, we use 個 & 間 more than 所. They are both correct undoubtedly, it's just the speaking habit.
while "Which school is he going to pick?" is a precise translation, "which school is he planning to go to" is more everyday American English
"what school ..." should be accepted too, although "which school ..." is slightly more correct/formal