Translation:How many pupils are there in that school?
To be clear "studento" is specifically a college/university student. For everything before that (elementary school, junior high/middle school, high school) "lernanto" or "lernejano" is typically used.
"Lernanto" is much broader than just a school student, by the way. It can also be used for participants of a small course, etc.
If lernanto is "pupil", could one use "lernonto" for a person which is about to learn? For example a young child, which will begin to go to school next month? And could i use "lerninto" for person, which do not learn anymore, e.g. people which already have left the school/university?