Well firstly you have too many classrooms. Bulldoze the school and leave the university and all students will go to the uni instead. Students don't have grades in SimCity.
Secondly it appears that a neighbouring city has sucked up all of your students in their high school buses and they can't return home. I fixed this by bulldozing all school bus stops so that the neighbouring cities don't steal my students, I then bulldozed each residential zone one by one and they rebuilt with fresh students. Then, all of my students went to my university and I fully upgraded it.
The end <3