Connecting multiple HC-06 bluetooth modules to multiple smartphones at the same time
It's hard to come up with a definite answer without additional clarification, but one possibility is that the bluetooth modules didn't show up because they had already connected to a phone that was paired with them earlier. 

If you used your phone to pair to each of the bluetooth modules before class, they might all have connected automatically to your phone on powerup, and then they wouldn't be visible in the list of available devices to pair to. 

Your phone may have reached the limit of how many devices it can connect to before connecting to the 6th device, which would explain why only one device was showing up. Once you try to pair to it, your phone would have to disconnect from another module in order to connect the new one, and then you would see a different device show up in the pairing list.

I recommend that you try turning the Bluetooth on your phone off completely before turning the devices on (and anyone else phone that has paired to the modules before), and  then have someone scan for devices with a phone that hasn't paired with them before. You could also manually try to unpair from each device on your phone, but the method just described is less likely to have a problem come up.