Don't know if it will help, but you might have a couple catch on and they do it on their own time, so no danger in it. Challenge them to a game called FT golf. You shoot 18 shots like holes in golf, and you score it as a make is a par, a miss is a bogey and a swish with no iron, none!, is a birdie. Show them the game at a practice and send them off in groups to each try a round and see who's best. You can also do it on your own and tell them your score; some will want to beat their coach. It's a fun way that you might get a few of them shooting FTs on their own and focusing on mechanics and makes. I was never a great shooter, but it really helped my shooting because I was focused on making the perfect (swish) shot. Amazing how many more "off" shots started going in. Sometimes I'd get frustrated that I didn't get a great score, and realize I had made 15 or 16 out of 18 FTs. When I was practicing, it got me well north of 80%. My best was -6 btw! I think my son got to -9.