Kagen is right about the tax issue
The difference between the government giving you $100 and the government reducing your taxes $100 is pretty academic. You still end up with $100 more and to pay for it the government needs to collect more from other people.
I think there is more space to argue on the question of it violating establishment of religion or not, but that would depend on the technicality and practicalities of the program. A law like this could pass muster, if the program was actually fair and equal for all schools and the state insured that everybody had access to quality non-religious education, I don't see it as automatically illegal. In practice, I expect this falls into the same category as the voucher programs constantly being pushed around here. They are designed as backdoor subsidies to religious schools, and those behind the vouchers often intend them to funnel money away from public schools.
Jay