Because every education building is using students to fill itself, so I think the problem is your other education buildings are taking the students for themselves and theres no one available for university. I did a test yesterday and it really is that you only need the highest available education building in your city. So if you have university, demolish all other schools in your city. It's weird but it's how it works. Each new level of education building replaces the previous.
Also try to build more busstops
Bring out the education table and start placing schoolbus stops near the uneducated buildings and the enrollment will just sky-rocket and there will also be less risk of fire.
Do it for about 20min realtime and you'll see the effects.