So you have a bunch of RPi 3Bs in use as MoodleBox. The better choice for the field in my opinion: 3B+ consumes more power but no proof that it can serve more Wi-Fi clients.
The issue is that sporadically MoodleBoxes fail to transmit wireless. Then don't recover until you reimage the card.
Well, random failures are the worst kind do diagnose. Basically there are two routes. Either to "guess" the cause and make the error reproducible by simulating the condition or find a failed sample and do a "post mortem". I would go for the latter. Connect your computer to the ethernet side of a failed MoodleBox, log in to its shell, use the power of the CLI to analyse the state. I know, in the field there may not be (wired) LAN equipment. Due to this I always buy a laptop which has an ethernet interface and have a network cable in my bag. Until you get the hang of it, connecting to MoodleBox ethernet 1:1 could be daunting. I start a DHCP server on my laptop, transmitting only to the Ethernet, plug the MoodleBox and scan the IP address range of the DHCP server.
Nicolas , may be you could add a routinge how do diagnose the Wi-Fi in MoodleBox?
P.S. I never had the impression cold starts and stops are a problem for the Raspbian. By any chance, do your people "hot swap" the SD card? ;-(