According to federal law, you can only discriminate in hiring if the requirement is a genuine issue. Jobs such as secretary, nurse or accountant can be done equally well by men and women, so specifying “female only” in the advertisement could get you in legal trouble. There are exceptions made for religious organizations for some requirements, such as having a man to teach gemara. For secular studies, I’m not sure what the law is. Since the job of secretary in a yeshiva could be performed by either a man or a woman, it would probably be illegal to specify a woman.
Some frum employers get around the law by advertising only in frum venues, or just using word of mouth. And I agree that it seems that some people hire girls and women precisely because they can pay them less – but that’s been a problem in the secular world too. Employment law is complicated, and needs a legal expert.