High density wireless -- pros and cons of different wireless systems?
There are a lot of variables involved. Questions to ask yourself would be:


How many RF channels does the protocol allow?
What is the network topology? Can you get away with multiple masters or peer-to-peer     networks? Can a master scan RF channels if a master/slave implementation is needed?
If you are using a off-the-shelf system how many nodes or addresses are allocated to the protocol?
How much data does each protocol allowed to be sent in a packet PLUS the overhead data the protocol needs to send?
How fast do you need to send data?


If you take all of these together you can play a numbers game to see which one would work for your classroom. Maybe a couple of masters with only 10 or 20 devices could get the data rate you need on separate channels?

A quick search on Google shows XBEE has 16 channels and can have a 16 or 64 bit address. 

Problems that could arise if these weren't being considered:


Flooded bandwidth
Slower data rate
Not enough addresses for all of the devices (anything over 5 bit addressing should cover it though)
Interference from an outside source such as another network or something like a microwave