I'd assume the log level is set to default (=minor) so might be a good idea to set it to informational so you'd might get more informations next time.
GUI > Controller > Log > Logs Configuration > System Log Level > Wireless AP Log Level > Informational
I'd also strongly recommend to upgrade to V9.21 - V8 is very old and the last patch was released 6 month ago.
One last question - not related to the message - are all the topologies on the controller configured to bridge@AP or do you also use bridge@EWC or routed.
Topologies are the available interfaces that you'd use to forward traffic from the WLAN to the LAN.
In the Role section you'd set which Role should use which Topology.
In that case you've 2 (the Admin is just so you'd manage the controller).
I was asking because the controller has only one interface (esa0) connected/up and was concerned that the whole client traffic of the school is passing via this 1GB interface.... which wouldn't be a very good design decision 🙂