Mice

Most people won't steal something if you make it too time consuming to get it loose. My reccommendation is to set the mouse up with enough slack in the cord that it can be placed on either side of the keyboard, then zip tie the mouse cord tightly to the legs of the desk or some other heavy structure, spacing several zip ties widely enough apart that they would have to be cut seperately.

Or you could play real nazi about it... and lock up all the mice and make the student's sign them out if they want to use the computer.

Or make them learn the keyboard shortcuts like everybody else ;)

...and cannot vouch for their product, but there are mice locks out there:

http://www.mouselok.com/

A watchful eye and suspension.

Anything else just becomes a challenge.

You could switch to optical mice that do not have a trackball and then use the previous suggestions about tying them down.

Another option is touchpads like those on laptops. My old high school did this before optical mice became commonplace.

You can use staples or U-shaped nails to fasten down the cord so students cannot remove the mouse. These nails can be found at any hardware store.

I know it may sound weird, and this certainly could pose an issue if/when trying to clean the mouse ball/rollers..

A small dab of plastic cement or "Crazy Glue" just on the tab of the retainer would keep the retainer and ball in place (of course make sure not to use too much that it gets on the ball).

Otherwise Optical Mice would be the better bet. The price has come down quite a bit on them if you had to replace a large amount (check with surplus websites for great deals on OEM or "white box" mice).

As far as for the entire mouse itself, I would agree w/ the cable staples to tether the mouse cord (ensuring enough slack for the mouse user) to the desk. Either that, or the zip tie effect would work as well. If you couldn't zip tie the cord to the desk, try zipping the mouse cord to other cables (keyboard, monitor, etc).

Hope these suggestions help.