Iguana as classroom pet? bad idea. Not only are they quite large and expensive to take care of... But they are "unstable" as far as personality goes, even the tame ones can and will bite and thats not what you want if you want students to touch the reptile. If its for display ONLY I'd say iguanas are a possibility but students in and out of a room all day can make them a bit stressed out. Proper habitat is going to be 6+ feet wide x 4 feet deep and as tall as it can get - a lot of room out of the classroom.
Your best bet is a smaller reptile that easier to handle and maintain - Bearded Dragons. These guys are very common and available at most pet stores. They are well tempered and many seemingly enjoy human interaction. a 40ft "breeder" style cage is sufficient, these guys are omnivores, as they reach later stages in their lives they gradually become more and more vegan.
I am also a personal fan of Uromastyx, these guys are quite timid and shy but very cute and well. They a 4ft plus wide cage x 2 feet deep 2 ft tall - minimum.. And its not just because of their size but the thermal zones, your talking 120-135F on basking end all the way to the mid 80s on the other. These guys are going to cost you more than the beardies both in cage setup and the animal itself (depending on subspecies). They are completely vegan. The subspecies U. Geryi (aka Sahharan uromastyx common name) is probably going to be the most tolerant in a classroom & less pricey than many of the others. Some of these guys get more stressed than others as well as differences in tolerances among other subspecies (there are many). I would NOT buy uromastyx from pet stores they will most likely be imports full of parasites - you would want a captive bred.
There is a club simply called uromastyx club on facebook with a lot of research data on the files section, and people are more than willing to help others on there. If you choose a uromastyx I'd browse that page on facebook an adult/young adult (these guys will live a long long time dont worry if its adult) - You will want to ask people the tolerance of the reptile around peoples movements as it will be a classroom. Theres another uromastyx page called uromastyx classifieds if you want to buy one.
Bearded dragons are likely going to be your number one go to animal.
I would like to say as a teacher, please do your research and pass down this information to your students I think if you bring in a reptile it can have its advantages for sure - BUT youy have to consider that whatever reptile you get those kids are going to go to their parents and insist they want one... So it becomes extra special to teach them proper husbandry VS the stupid ass petstore bandwagon advice they give people. Reptiles are brought into family homes where they are not taken care properly and suffer. Conditions such as metabolic bone disease and renal failure are VERY common as pet stores sell these animals and their employees typically have no idea on advice beyond selling them (typically wrong) equipment. OFten times if they are not yet dead and the owner decides to give them up - rehabbers shell out a lot of their own personal money to fix the problem. There are plenty of regular visitors to this forum that can certainly share their stories on this lol.