What symptoms did she have between the concussion & Dec 2017 constant dizziness?
Was there another head hit, hard fall , sport injury or any other thing that might have set off or caused the increased dizziness?
It seems like a long time span for the constant dizziness to start up.. without some sort of triggering event..But I could be wrong on the time span..
Did she lose consciousness at all with the original injury?

I suppose the doctors are also ruling out other causes or illnesses? Just in case..

If school becomes to much for her there are online schools that are free just like public schools, she can log on for short time frames to do study/classes - whatever fits best for her ability & fatigue levels.
Pushing thru fatigue & symptoms can cause set backs & delay healing..

might look into Nucca /upper cervical (c1 c2) chiropractic.. Up C Spine has very good info on the subject.. this does not show on standard imaging & MDs don't really look into it.. IMO the old bias of MD vs DC... a very good DC can be very helpful..