If the head stays up, then the bum stays down, makes it much harder for them to buck. Maybe try this a few times until he begins to relax about working away from other horses and then try riding as you usually would?
Try shifting your weight backwards. It's harder for the horse to buck and maybe it will help her stop. Also try to ignore it then they'll hopefully.
My horse does the same! With mine if I keep him moving forward he doesn't buck, I've found that if I do transitions a lot he starts listening and realises he can't get his own way, good luck with finding a solution 😊