"It varies" is such a fucking unhelpful answer. You can still give examples and ranges to give people some idea what they're about to go through. When I was looking up what it meant to be a teaching assistant, every website was like "it varies" with no further elaboration. I can give a better answer than that in a minute!!
At a secondary school, you'll most often be accompanying one pupils or more during their classes, keeping them on track and helping them with their work. You'll likely be swapping pupils each class. At a primary school, you're more likely to be generally assisting the teacher. Even if you're accompanying a pupil, you'll likely be running some lessons/sessions with the children by yourself, preparing the classroom, and doing odd jobs the teacher needs done. If the child is older, it might be more like the secondary school option.
EVEN IF you go to a school and get a different role, it's not going to be SO off the wall that that advice above is useless! People, learn how to give better advice!