At the places I’ve ridden at, I’ve always been told, very firmly and happily, that there’s no conflict between boarders riding and lessons, but that’s never been my experience in practice. Beginner lessons tend to be, well, beginner-y and steering is an issue. More advanced lessons demand greater use of the arena, and then all riders have to come to a standstill while the riders in the lesson jump the course, do lateral work, or sort out a green horse’s issues.
Even when I had the ideal–a large outdoor arena and small indoor arena, so I could at least pick the one not being used, on dark or wet days, there isn’t really a choice. Finally, there’s the issue that some boarders don’t like riding with one another, because they don’t think the other person uses the space respectfully, or other barn drama.
I would love sign-up times, but the one instance I was boarding at a private facility where that was done, the times weren’t honored.
At minimum, when there’s lots of arena sharing, there needs to be specific, agreed-upon policies about passing, slower and faster riders’ use of the rail, jumping (or not) out of lessons, and so forth. At the very first barn I leased I horse, there was a rule about not cantering when a non-cantering rider was in the arena. I didn’t know this, and was read the riot act after I violated the policy (which I obviously wouldn’t have done, if I’d known about it, but the owner assumed this was a universal rule, like passing left to left). Most barns I’ve ridden at want everyone going in the same direction as the lesson rider unless someone is jumping, but obviously this isn’t universal. The rules don’t need to be posted, necessarily, but it shouldn’t be assumed that all riders know them. This also goes for changing the jump course, taking down or rearranging jumps and poles by non-trainers.