Horror movie title from 60's/early 70's about a boarding school?

I watched the beginning of this movie about two years ago on Halloween eve on AMC, very late at night. I fell asleep and could not find the title of the movie so I could watch the whole thing. The movie was possibly English and was about a woman who accepts a teaching or head mistress position at at boarding school. The school and all involved are a bit weird and the woman is increasingly suspicious. I think the school was a front for either a satanic cult or a coven of witches. Anyway, I have tried searching every single way I can think of with no success. The movie was NOT called Satanic School for Girls or The Covenant. Any help is much appreciated!!!!