Actually, as long as a school produces good academic results, And no one is forced to go there (other than by there parents), I don't care. If they have parents that would choose that religious school, the children are going to be brought up religiously, regardless. But being allowed to choose a school whoise teaching style matches the student is the best recipe for overall success. Too many students are trapped in failing, sometimes dangerous, public schools because there parents have no choice.
Because you mentioned religious schools, I'm going to assume you were addressing vouchers, rather than charter schools.
Charter schools are actually public schools that are run privately with more flexibility, to better meet children's needs (note: these usually cost LESS per student than standard public schools). Charter schools work closely enough with the government, that it is easy to prevent them from being religious schools. Charter schools also can (and have) been shut down if there are problems with academic results or questionable finances.