Because the filthy, perverted lifestyle that you're raising your daughter in, and the horrendous 'family values' that you are teaching her, flies in the face of everything being taught to her in a Christian school.
How can this "mother" want to put her daughter into an environment that firmly teaches that her mother's entire lifestyle is sick, sinful and evil? The answer is that she doesn't want her child there for any other reason than to attack the teachings of Christianity in a Court of law and to perhaps make a windfall profit in a lawsuit.
In other words, she uses her own daughter as a platform to spit in the Face of God.