I had the same problem for the first 6 monthys of having my horse & This may sound really stupid, but it solved the problem in my 23yr old tb mare...
I pulled the bit off completely and put her into a hackamore, it took a little getting used to for her, she was confused as ever, backing up instead of going forwards, left instead of right, you get the idea, but after 20 minutes of quiet, soft attempts, and we were jumping in it the next day, she is now better than she ever was before.. she also used to bolt like there was a wolf hangin on to her tail (funny that though as we dont have wolves in NZ lol)
But it has also stopped that aswell, and you say you have beginners/novices on her? I think the hackamore would be an idea to consider as then the riders won't accidently pull on her mouth and hurt her.
Good luck :)
This is bubbles now... 4th time being ridden in hackamore
first time actually going ovcer 90cm... before she used to refuse constantly.
^ Then when i dont release because im expecting a refusal, I don't feel quite so guilty about hurting her mouth. It's BRILLIANT
You can't see hackamore very well in these pics, but you get the idea :)