The best way to put it.. you have your iguana personalities.. Sometimes pure out hostile/aggressive (of which there are more of these than tame ones), "tolerant" ones and the occasional but incredibly rare attention seeking iguanas. They all can and will eventually try to bite due to it defensive, aggressive or hormonal (aka breeding season) problems. Now lets add the BIG time stress factor of placing an iguana in a school setting.. That is going to totally freak them out as Iguana's do not respond well to people traffic.
An adult iguana can inflict serious damage that can rip through tendons and ligaments on peoples fingers, break the bone on fingers etc.. And a strong bite WILL leave a scar forever. So lets take that consideration when it comes to students handling it (with or without your knowledge, and when I say without - I know I would have found my way into an iguanas cage without my teacher looking when I was in school). The award is simply not worth the risk. I've known countless people who have had their iguanas for years, then one day they are doing their business then SNAP goes the bite - for a teacher that would be a nasty situation to be in.
Just to add.. Taking the iguana (and any other reptile) to and from different environments during holidays etc is usually not going to be the best idea. They are territorial creatures by design, and taking them away from one territory into another kinda freaks them out to the point where they often times will not eat do to stress factors. They do NOT need the human factor so it's best off left where it is).
Like I said.. Having an educational reptile is a good thing when also properly teaching the needs of such reptile to the students. An Iguana? Too many issues. You could get a cage with 2 or 3 levels to have multiple types of reptiles/other types of critters for under the same cage size as an iguanas cage. I recently referred someone to a builder that made them a 6ft tall x 4ft wide cage, they placed a uro in the top a beardy in the middle and I think the bottom have they just used as storage but you can mix it up.. Like a beardy one compartment, maybe a tarantula, gecko whatever. I really do suggest bearded dragons though.. If you want a reptile that your students can handle its your golden ticket. Russian torts are also a very good choice.
Oh and while I am still trying to sell you to go bearded dragons.. They have a blood borne backteria (I cannot remember the specific name of it) that they are for the most part, incredibly immune to but very deadly to other reptiles. If you get one and there are other reptiles nearby, be careful of cross contamination. Someone on the uro forum was complaining to an exotics store that they clearly saw a uro stricken with this bacteria and yet they were going from cage to cage without washing their hands.. Basically every reptile there should have been isolated and not sold because of this simple mistake.