I work at a lesson barn that also has chickens. Any of the heavy breed hens would probably be a good choice-- they tend to be gentle and not flighty. I wouldn't recommend leghorns-- in my experience, they live up to their reputation as flighty little freaks! My Buff Orp hens, Barred Rock Hens, and White Rock hens are lovely to have around, though.
Unless they're dead set on having a rooster, I'd probably recommend not having one. In my experience, whether a rooster is aggressive or not is a TOTAL crapshoot-- you can get an agressive one in even the breeds known to be most gentle, and a gentle one in the breeds known to be agressive. For example, my Buff Orp roo has come after me occasionally, while my RIR rooster is the sweetest, cuddliest thing known to man.
All a rooster needs to do is go after a little lesson kid to cause a whole host of problems, and a lot of those kids are LITTLE. One really could get hurt.
It's great to have the hens around the barn, though. Really gives the place a "farm" feel, when so many of the lesson barns have become so much more sterile and business-like.