First off, best option is to find TURNOUT for these horses. It's no wonder so many of them are aggressive and angry if they are stalled 24/7, and if a lot of them are youngsters, it's going to seriously screw them up for the rest of their lives, both with people and other horses.
I know it's not your decision, but it needed to be said.
Secondly, bring them outside. Surely there must be an arena or something they can have a run around in for just a short time? You could also build up to turning them out in small groups so they at least get SOME social interaction?
We regualrly muck out our stalls with the horses in them, but this is only because they are all quite happy for us to do this. I would never do it with a particularly young horse, one that was nervous when stabled or one I didn't quite trust.