I went to a strongly Christian grammar school in the 1970s. It was not a church school and there wasn't a nun or a clergyman in sight, but transgress the rules in any way and you were told that you had acted in an unchristian way. This view was usually put forward and promoted by the headmistress and her gang of four - the white, middle-class matriarchs who ran the school.
In fairness, we had quite a broad RE curriculum presided over by a couple of rather eccentric and decidedly non-conformist ladies who definitely didn't push any one religion at us. My teacher did actually say on a number of occasions that she could not tell us what to believe - only that we should learn what other people believe and then decide for ourselves. We studied a number of faiths in depth which, I think, was more in deference to the fact that the school population was made up of a good number of Jewish, Sikh, Muslim and Hindu girls, as well as a sprinkling of Jehovah's Witnesses. I remember we also discussed a lot of moral and ethical issues such as contraception, drugs, disability and homelessness.
But for all that openness in the RE curriculum, there was still very much a sense of the teachers expecting us to behave according to Christian principles. We'd be told off (and sometimes put in detention) for not closing our eyes and joining in with prayers, and for not singing (Christian hymns) in assembly, for instance. I also remember one girl, a Jehovah's WItness who believed in giving small gifts all year round, being hauled into the headmistress' office for giving ordinary, single pencils to all her classmates.