Sorry for the topic necro, but seems its a mixed issue of agents getting lost in mass transit and not having a school with in certain radius of there home.
I solved most of my issues by dedicating streetcars to the uni, and plopping a community college next to my low wealth mega tower, as that tower kept having 600 ish unrolled students. Despite the fact that all mass transit options have a wait time of less then 45min, some students return home around midnight (acording to the data layers). Streams of pedestrians are walking around my towns that have mass transit, so im thinking there walking home from one side of town to the other.
I have even seen agents walking from 1 bus or street car stop to the other, to be transported 4/5 blocks and then walk back agian in a congo line to rince and repeat .....
PS: i personally dont have the game, and had this happen on a friends game in offline mode when i wanted to see how bad it really was (after reading several reviews and watching tons of youtube videos). I dint buy the game because of the online requirement, and seems with this dumb agent system it was good choice would have been money wasted tbh most basic things dont work because agents are getting lost in the most simple road/transit networks not matter what you do.