New to the list. Choosing classroom fish

I have a 70 gallon freshwater tank for my classroom. What is the sturdiest, most forgiving fish to raise? I had four Oscars and lost three of them when the power was out for two weeks during a winter ice storm. The one that survived never was strong again, but he lived another two or three years. Three plecostomus survived the storms, and are all between 7 and 12 inches long. The room is always warm (barring another weather disaster), but in the summer I can only get in once a week, and even then I'll be locked out for three weeks. We need easy to care for fish that the students will enjoy. All advice is welcome.

Much as I love Cichlids, I would go more for a planted community tank in a classroom where maintenance may be minimal at times. Saying that I would probably look to be having quoted filtration that is double the actual size of tank. You can never have to much filtration, and if during summer you have to be away for 3 weeks it will allow you some peace of mind.

Thinking of fish. Depending upon how you wish to use the tank in your classes, I think I would be tempted to look at the live bearers. Generally small I know, but colourful.
Guppies are more mid to top of tank generally, Swordtails and Mollies all round the tank, both offer you colour, and the ability for easy identification of the sex of the fish.
Once content, they will breed profusely, hence the idea of being planted, so some fry may survive.
Chinese Loach is entertaining as it goes up the glass, how it behaves is very much hit and miss, some are placid, others....not so.
Pictus are wonderfully active, but note they are marked as semi aggresive (that means they gulp down smaller fish during the night as a snack).
The Barbs may be suitable, but I have no experience with keeping them, so someone else may have suggestions regarding them as occupants.
What ever you go for, i think it's a great idea, but I doubt anything is going to survive 2 weeks of no electric during an ice storm, not only the drop in temp, but also the lack of filtration. Hopefully you won't go through that again.

Barbs are pretty hardy from my experience. The only thing is that they love nipping fins, so I wouldn't put tjem with any fish with long fins

Here's an idea for those periods of time where you don't have access and its something my neighbor uses with great luck. It's one of those battery powered automatic feeders she's a run-a-muck camping fool and this was a great answer for her.

Planted would be a great way to go with some low light plants, a couple male kribs (they're pretty tough), platys, guppys (the kribs will keep the population way under control), personally would stay away from the tiger barbs but there are others that would be great, couple bushy nose plecs.

My 2 cents

Giant danios are another active species...cory cats they can gulp air if the power goes out plus look great in a planted tank

I agree on the Giant Danio's. I had a couple in a community tank, always active.

My suggestion would be small barbs!