Is the 去 before 上学 here necessary? Can't I just say 他们什么时候上学?
As long as 上 is combined with the hanzi 学, which altogether forms "to go to school", 去 is optional here.
What's the difference between using 什么时候 and 几点?
In English "when" （什么时候）is vague and could mean a day of the week or an interval, such as every Tuesday or from September to June. "What time" （几点） is a very specific question.
It might work the same way in Chinese too, but I'm not sure.
Xie xie ni very much CinnamonTe1
幾點 is for time (on the clock)
什麼時候 is for time (like an estimate or a before-after thing)
What's the difference between 上学 and 学校?
上学 is go to school, 学校 is school.
Why is "What time do they go to school?" wrong
Its When are they going to school.
How about "when would they go to school?"