Did you suggest visiting the school?
Maybe she doesn't think you will take her concerns seriously. Does she think you are being condescending to her?
It might seem silly to you but when all you have heard all of your life that you must not have anything to do with "false" religions, it would be easy to have a knee jerk reaction about having your children in a nonsecular school.
I remember when my daughter told me her class was performing for seniors at a church. My first reaction was no...it is in a church...church bad..
Lighting might strike. She assured me it was in an attached building not actually in the church.
I let her go but still felt guilty like I was being disloyal. We are always told to stay out of Babylon the Great.
I think someone must have mentioned a concern about the school. She might not feel comfortable talking about it because it's hard to explain to non JWs.