I have ridden several, including getting to lease one while I was a working student, absolutely priceless…especially the lease, as he was kind but very sensitive and very correctly trained. I love my young horse, but I would pretty much kill to keep taking lessons on schoolmaster to keep me honest.
For the most part, schoolmasters will do what you ask. Not what you mean to ask, but what you’re actually asking. If you stay relaxed and have fun, it’s no big deal, you can’t let it frustrate you, just laugh it off. Depending on age/tolerance level, it’s very important to remember the horse is teaching you, not the other way around…some of them will get pissy if you keep (accidentally) telling them to do something and immediately correcting them.
Some of my struggles (different issues with different horses): passage instead of canter (really a lot of things instead of canter…the canter transition has to be finely tuned b/c of the changes, so until you figure out the right set of aids, not so much canter!), once you get the canter, be prepared for tempis of various counts…when I started that lease I could get canter only from the walk and on a circle…if we went in a straight line it was 1 tempis…then once I figured out how to keep him straight and on the correct lead, I couldn’t get a lead change on purpose! On one horse I kept getting accidental extended trot to accidental piaffe…almost sent me flying the first few times. At least I’d sort of figured out the half-halt, just had to do less.