In my experience, parochial schools do not exclude on the basis of religion. They teach (their) religion as a part of the curriculum, but they allow children of any religion to attend. It’s not a matter of exclusion, but the teaching of religion, that makes a voucher system unconstitutional.

And the point that’s being ignored, or missed anyway, is that vouchers will inevitably take funding away from schools in low-income neighborhoods due to their increased transient population.