‘frei’ means Jewish, not religious/’nisht frim’.. to the best of my knowledge.
I’ll never forget, when I went to enroll my daughter in a BY kindergarten: the administrator said to me: ‘don’t worry, we don’t have the frei here’ (or something to that effect.) that was about 24 yrs ago.. I had been a BT for only about 12 yrs. I had gone with another mother (also a BT), & neither of us enrolled our daughters there. ironically, our girls did end up there a few yrs later.. there was the most wonderful administrator in the former’s place! it was a gr8 school, btw.