I find this problem as think out of the box problem and the hint is here:

The students have no way to communicate with each other in any way from their rooms.

ie; They can communicate with each other out of their room. 

How?? Each student can drop their pen or draw a line (only once) on wall. If there are 22 pens or 22 lines and if the student taken to the play room has not dropped the pen or drawn a line, s/he can scream.

"Switch" is the diversion of your mind from the main problem.