chayav inish: you cannot put pressure on others and be cheap and call it chessed. high school girls are often loaded with responsibilites from school and home and are cutting into their studying/free time to help you. and i believe it is a chessed even though they are paid. aside from that, many of them are saving for seminary/camp/for after they get married (yes i actually know girls who babysit for my kids in all three of these situations.)
Baruch Hashem, many girls are raised these days with a sense of modesty and refinement that does not allow them to “name a price”. i often have to really push and they wont even tell me how much they usually get. “whatever you want to pay is fine! really!!” I think the fair thing is to ask around before you hire so you don’t inadvertently make a chillul Hashem or a bad impression, esp. if you are a kollel family