Troll Touch has been manufacturing after-market touch screens for Apple computers for years. Now, the company has added the latest slim iMac to its list of supported devices.
You can get a resistive overlay that's self-installed or have the folks at Troll Touch install a built-in touchscreen on either the 21.5- and 27-inch iMac. Troll Touch is an authorized Apple service provider, and can perform in-warranty repairs on iMacs with the factory-installed touchscreen.
Years ago when I worked at a school, we had several machines with Troll Touch screens, both the overlays and the in-machine units. The latter are better suited in environments where the kids might remove the screens, of course.
Pricing on the user-installed model was unavailable as of this writing, but the factory-installed option will run you US$995 for the 21.5-inch model and $1,295 for the 27-inch iMac.