You and your ex-husband can agree to a plan that best meets your daughter's needs, however. Try to talk with him when you are both calm and explain your concerns about your daughter's need for routine during the school week. If he insists on having her during the week, you will have to let her go.
After a few weeks of this routine, talk with your ex-husband again about the situation. Your daughter may have adapted and may be doing fine with it at that point. If she is unhappy about it, her father will probably realize this, in which case he will most likely be willing to go back to weekends only. If she is also having problems with it, ask for a conference with the teacher, your ex-husband, and you. The teacher cannot take sides, but can share information about the effect the plan is having on your daughter at school.