Frank, I can share our experience in West Africa with RPi2 using USB WiFi dongles. They seem to work well with up to about 20 clients connected for accessing normal “RACHEL” content and even streaming Ka-Lite videos, but if students log in (which activates performance monitoring/logging) and access the math exercises, the system becomes VERY slow and many time-outs/disconnects if more than about 10 students work concurrently (Sometimes it chokes with as few as 6 connections). How much of this is due to the AP USB dongle and how much to the under-powered RPi processor is hard to say. We “upgraded” to the RACHEL Plus Content Access Point which Intel claims can handle 50 connections, and we find it exhibits the same symptoms when we exceed about 20 logged in connections doing Ka-Lite exercises, but it can stream to over 30 clients not logged in. Hope this helps you in your project.
Leon