RE: water resistant HT

Unfortunately, unless your students are HAMS, you would not be able to use any HT's from the HAM world.

But what you can do is use the bew Family Radio Service (FRS) HT's. These units require no license and run about 300 to 500 mWatts (almost 1/2 watt). In a open area they can cover up to about a 2 mile distance. More reasonably they will cover 1/4 to 1/2 mile in terrain that has folage (trees) and buildings.

Some of these units are built pretty tough, as they are intended for the general public that does not have the same attitudes toward radio as us HAM's do.