We have 2 sets of 20 bluetooth keyboards for class sign out. Since it becomes an inventory nightmare trying to track down individual keyboards, we only allow the sets to be signed out.
We are a 1:1 iPad school w/devices deployed from 5th-12th grade (approximately 800 total iPads when including teacher devices). As part of SBAC testing, we had to provide a keyboard. The issue with pairing and unpairing was relatively minor. The day before the testing, we went to each classroom that would be using them the following day. We had each student pair up with a keyboard and note which keyboard they used (based on inventory label on the back). The day of testing, they only needed to make certain that they grabbed the same keyboard.
We're using model K1280C made by Anker.
I don't know if the problem is specific to this make/model but they are battery hogs.
If you have a number of left-over USB keyboards, a short term solution we tried was to plug them into the iPad using the camera adapter
(Apple iPad Camera Connection Kit - Apple Store (U.S.))
The keyboards worked fine for about a week. Then, they started to become unresponsive, even when plugged into a standard workstation. I can only guess that the power they were receiving from the iPads was either not consistant enough or too little. Either way, the keyboards all ended up in the ewaste pile.