I agree that riding lessons and clinics are very low Covid 19 risk for the participants, but the risk is going to come when people are off their horses, spectating or auditing or hanging out by the catered lunch.
Our province just suspended adult recreational team sports, but not children’s sports. The reason was that adult teams spend so much social time together. There was a big outbreak with some adult beer league hockey team that traveled for a game.
Anyhow, international clinics all got cancelled because if you travel you need to self quarantine for 2 weeks in Canada at least.
People are still giving local riding lessons and I have not yet heard of a transmission event at a barn in North America. Jockeys in dorms, yes. But not riding stables.