N.R.
This happened with my ex also. Unfortunately you can't force someone to be a parent. My kids father disappeared 5 years ago. My oldest was 4 and the youngest 4 months. I took my oldest daughter to counseling because of her declining behavior. She was a daddy's girl and he bailed, of course she was going to act out since she was too young to know how to handle it. It took about 2 years for my daughter to learn to deal with the situation and even now at 10 years old she still asks hard questions and her behavior becomes difficult because of it. My advice is to go to the school and talk to the counselor yourself, maybe they have a mentor program where she can have a positive male role model. I wish I had more advice, but it may get worse before it gets better. Your daughter has to learn what works for her in dealing with the situation and every kid is different. I got my girls into Girl Scouts and community theater and it has helped a lot, that may be something to think about. Give her an outlet so she doesn't just think about it constantly. I wish you the best.