It can be as artsy and complicated, or as simple and easy as you want it. It's just a notebook that you turn into a diary that suits your needs.
Does she already keep a diary/ planner? If she does, or has shown interest, then it might be a nice thing for her to try.
Mine is now mostly used for work and for tracking birthdays. I have the dotted Leuchtturm journal because it has three ribbon book marks and a little pocket in the back, and it's hardcovered and handbag sized.
In mine, I have the year drawn up so I can add future events on a long term plan. Then I do a weekly page. 6 x boxes on the left side of the double page (weekends go into one box) and on the right page, a little box bottom right to plan my evening meals and the whole rest of the page for my to-do list. All pertinent calendar events from my work online calendar are written into the boxes, so I start the week with a clear idea of what's happening, just because I've reviewed and written it down physically.
I use the blank pages afterwards to make meeting notes or plan stuff that happen that week, so I can easily find minutes/ info. Then when the week is over, I draw up the next week's double-page.
There are loads of guides on Pinterest on how to do bullet journalling for school. She might find it useful to have a look- things like class notes, assignment deadlines, reading to do.
@teawamutu, when I first started, I tracked exercise and sleep and water intake, but I have a fitbit watch that does the first two, and I don't actually care about my water enough to colour in little boxes. Now, I don't use it to track things like that. It's got to work for you and fit into your routine and life. Otherwise it becomes a chore!