Keep in mind that a policy on jumping outside of lessons doesn’t necessarily need to be one size fits all, as long as it is clear and fair. I ride at a barn that requires anyone under 18 to have adult supervision present when jumping their own horse outside of lessons and strongly encourages adults to have another person present. Now, sometimes that person is a parent or friend who doesn’t have tons of horse knowledge, but at least you have someone there if things go wrong. Those riding lesson/part leased barn-owned horses or paying for a practice ride are not allowed to jump outside of lessons without obtaining prior permission each time (so the answer may be no if the rider is not ready to jump outside a lesson or depending on the horse’s workload etc).
You could also consider periodically offering some sort of clinic for your boarders that they must attend once in order to be able to jump outside lessons - a few hours where you review safety, types of jumps, how to set jumps, how to develop a weekly or monthly training plan for horse and rider including amount of jumping vs flat days, etc.
If you typically have a course or exercises already set in the ring, you can also make a rule about not moving jumps except for possibly changing height, which will solve some of the potential issues like people setting bad distances. You could also allow people to only go up to a certain height (with a mark on each set of standards at max height.) This also allows people to practice the exercise or skills they were working on in lessons, provided you leave the same exercise up all week. That may depend on the variety of skill levels you have and how much you change up the lessons. Also if you have different disciplines and only one ring it can be hard to leave jumps up all the time.
