Primary Teacher Undermining Parenting?

Talk to the teacher, she/he will not know of your concern until you tell them. It is hard, as a teacher, to deal with students like your daughter. If she is the only one being kept back, then the teacher probably feels that by keeping her in, she will complete all the work and gain an incentive to speed up. The rest of the students can't just sit there and wait for her. If they were given a little activity to time-fill for her to finish, she would be still left out. I think you need to talk to the teacher and I hope that you have done this on some level if you are already considering taking her out.