Hello-
Another option all together that can still measure many students at once is an InstaPulse. It is a hand held pipe, similar to a track baton. After a good workout, or actually any time during the lesson, students can just hold the sensors on the baton and it reads their pulse in about 5 seconds.
Hope this helps!

Any idea what the cost of the Instapulse is in comparison to heart rate monitors?

About 100 bucks! Pretty cheap compared to HRM's