When I was in 4h (this is not super recently LOL) we had to have horses out of full time training by x date, but could continue to take lessons. Horse could also not be rode by anyone other than the exhibitor unless it was posing a danger. You should be ok to take lessons? I can't see them implementing a rule that discourages learning, education, and ultimately better horsemanship.. That's kinda the goal?
If you're pivoting by rocking back as others have suggested, you'll be pivoting on the "incorrect" leg (left rear planted). Incorrect being a loose term, as it's not pre-defined as to which leg should be planted in most rule books. If your horse has been doing it for 15 years (at which point left was considered correct and rocking back was common), and he does it on that leg and that leg only then its not something to mess with. But, if you're starting fresh and going to spend the time to teach it, do it correctly (forward around the R hind).
Don't worry, you are already moons ahead of the kids whose horses are still in the field ;)