I had the same issue today with MoodleBox on a RPI 3 B+.
This was a new install of the latest supported Moodlebox. I noticed that about 14 students could connect. No more could connect. From what I have read, this seems to be an issue with the wireless capabilities of the RPI 3 B+, and it is suspected that it is an issue that might be solve with coding.
Here is my question, in the meantime, would it be possible for me to get around the issue by simply plugging a wifi dongle into the RPI?
I even have a spare router in my basement that I could bring into the classroom and hook up somehow.
I must admit, I am pretty new at this.
Finally, would it work if I were to just take the microSD card out of the RPI 3 B+, plug it into a RPI 3 B, and try that? I have heard that the RPI 3B might be able to handle 25 students without issues.