Let's look at what went wrong here.
A fish most posters believed could not be aggressive was aggressive. Most online research will say that they are community fish, clearly. Most experienced fishkeepers here said that earlier in the thread.
The teacher did not have the Latin name, and used the trade name. I'm sorry but a minority of people who post here know the real names of their fish, and go with names of local stores or importers. Confusion with trade names is a feature here, and I have been criticized for using internationally recognized names because they aren't English.
If I hadn't had a psycho Sewellia, I would have accepted they're peaceful. If I hadn't seen obscure video on adaptations to fast water, it wouldn't have crossed my mind that current could control aggression. It never occurred to me when I had them., only after when I saw current used to control aggression in Geophagus argyrostictus, a rowdy member of a generally calm Genus.
So, if the teacher looked up the fish, they probably would have done what was done. The Sewellia were observed attacking, a legit observation.
If it had been a common pleco, how many of you would honestly observed they are often found close to outflows?
I don't think the hillstream loaches actually were seen aggressing in this case? He said they thought that because they had some losses after getting them, but that they'd bought other fish too. As far we know, there could be a small snakehead in there, going to town. But, it's a long thread, and I may have missed something.
You're right that even if he'd done in depth research, most sources would say peaceful, not a problem in a community, so he's not at fault even if they are the problem in the tank. But I thought we were now talking in general terms about teachers being busy, so relying on faulty fish store advice is okay.
But, I'm breaking my own rules there by being harsh I suppose, since if a beginner comes here wanting help because they relied on bad fish store advice, I always say that it's understandable, that it makes sense to trust that the fish store people should know what they're saying, of course most people will listen to them. So... I dunno. I don't even believe in my own point anymore... I just wish more people would care about looking into the animals they keep, before buying them. I've also seen some rescued snakes that were abandoned classroom pets and had been horribly, permanently deformed as a result of completely inappropriate conditions, and the fish that some science teachers have used as part of some cruel 'experiments', where they stick a feeder goldfish or betta in a cut off plastic bottle and stick a plant in the top, type of thing. Some really upsetting classroom pet stories out there.