I had the same problem. My solution is a USB WiFi adapter. For this I switched off the internal WLAN.
It works with EDIMAX EW-7811UN Wireless USB Adapter and TB® Ralink 5370 USB Wifi Dongle. When I#m working with the Ralink Dongle, I had to adapt the hostapd.conf file.
I works at least with 17 clients, Before that there were only 10 clients.
In the next few weeks I will try a larger class with more students.
Here is my configuration in german.
Konfigurationsänderung Moodlebox mit USB Stick
Zugang per ssh (z.B. putty). Raspberry mit Internetanschluss über lan.
Update
sudo apt-get update
sudo apt-get upgrade -y
sudo apt-get dist-upgrade
sudo nano /boot/config.txt
Folgendes am Ende ohne Anführungsstriche einfügen.
'# turn wifi and bluetooth off'
'dtoverlay=pi3-disable-wifi'
'dtoverlay=pi3-disable-bt'
Neustarten
sudo shutdown -r 0
Edimax
Läuft ohne weitere Veränderungen
Ralink
sudo apt-get install firmware-ralink
Änderung von hostapd.conf
sudo nano /etc/hostapd/hostapd.conf
Dann folgenden Eintrag ändern:
'# Enable 802.11n'
'# ieee80211n=1'