Disclaimer : I work for a school district where I know of about 5 science teachers with various strange living things in their classrooms, but I'm not a teacher.
For starters -- when looking at the whole "distraction" thing, the simple issue is those teachers that have problems with distractions are pretty boring. The guys teaching with SW tanks, lizards, etc usually are good teachers and don't suffer from students drifing off to space all too much.
I mean, if you love teacher so much that you will bring a tank into the classroom, you probably are not reading overheads in monotone
Secondly, most of the tanks and terrariums get partially maintained by the custodial staff and other staff that DO work year round. In one case, there is a 220 gal FW tank in the entry way to the school. One of the custodians tenks to it during the summer because he likes it.
In other schools, they will take care of basic feeding and check on the tanks every day or so (because the teacher is nice to them).
I work year round, and take care of the tank at our maintenance building when the owner of the tank is away.
The other problem -- HEAT -- during the summer the air conditioning doesn't run at all. If your district has a "salmon" program, you can borrow one of the many, many chillers that would go unused over the summer.
You might just get lucky and find out one of the year round staff is a reef head, and that solves most of the problem of the summer months. In the one larger school, one custodian is a reef head, and the other loves snakes -- so they all take lunch in the one science room with about 5 tanks/terrariums.