This really comes back to projectors with support for wireless networking, although some projectors allow tablets (and phones) to be directly interfaced by cable. Obviously in a classroom if the use is for different folks to present, then wireless is far less hassle.
At this point, many wireless networking projectors now offer free apps for presenting, or annotating from tablets. Some also allow command and control – the ability to run the projector, access menus, and to change settings from app equipped tablets and phones.
Note that capabilities can differ greatly from one app/device to another, so make sure if you are going down this path, that you can get the capabilities you need.
Slideshow capabilities are pretty standard, but whether you can see applications on a projector that you have running on, for example an iPad, will depend on what the tablet and app offer.