As someone with a few classroom pets (including snakes), I would say that taking consideration for the type of food required would be the first thing. Getting live mice is a pain and an expense... raising them is a smelly process... and co-workers may not appreciate frozen ones in a communal freezer... etc...
the next thing to consider would be weather the animal could tolerate the inevitable neglect that may occur due to circumstances beyond your control... sickness... school closings... power outages... helpfull night watchmen unplugging things to save electricity... etc...
after that try to anticipate all the things that could go wrong... escapes... deaths... regurge ... breeding durring school wide testing.... etc...
and finally, I would suggest not keeping anything in the school that would cause you to loose sleep wondering if you will find it dead in the morning...
... now that thats out of the way, my top pics for classroom herps....
small / med tortoises (like redfoots)
box turtles
corn snakes
rat snakes
king snakes
bearded dragons
leopard geckos (if you get them off crickets)
well established captive bred adult ball pythons
Herps I'd avoid
Pretty much all the arborial boids (dang!)
True Chameleons (other than captive well established veileds)
anything requiring high humidity
anything nocturnal (except things that adjust in captivity)
cricket eaters
anything with a septic bite (large monitors)
these are just my personal thoughts... food for thought and all... and I break my own rules all the time LOL
However, It would definatly be cool to have something that utilizes the height of the cage. I know from personal experiance that a large yellow rat snake would be all over the place (especialy when ready to feed) and a pair of bearded dragons would love sunning on a nice perch...
... I have seen alaborate vinewood structures for monkey tail skinks as well, but have never kept them personally
looking forward to updates and pics.