12 years of Catholic school.
1st grade: Evil-tempered nun, who would slap you for misbehavior or even for making a mistake.
2nd grade: Lay (non-nun). Quite nice… though once, a classmate got a bad report card, and she sent that kid down to the 1st grade nun to show her (really? WTF was that supposed to accomplish??) - and the first grade nun slapped the kid hard enough that she knocked her down.
3rd grade: Nun. Not especially evil, just aging
4th grade: Nice lay teacher, but one class with a nun who could be nasty at times. She actually had a reputation for having been worse (like one tale that she’d whacked a kid with a steel-edged ruler, that cut the kid, and the cut became infected). We didnt like her but she wasn’t as outright terrifying as the first grade teacher.
5th grade: One lay teacher, one nun. she was occasionally grumpy but not outright evil
6th grade: Both lay teachers. Mine was quite strict - but had a sense of humor about it. Had a reputation for swatting kids with her yardstick which was called the “happening stick”. Every year it would break near the end of the year, and the kids would chip in their pennies and buy her a new one. All the “good” kids (i.e. more academic, and/or well-behaved) loved her. Even the “bad” kids (the ones who struggled more, or had behavior issues) liked her OK.
7th-8th grade: a mix of nuns and lay teachers. Several were old and cranky, but not evil. The other one… well I got the distinct impression that she loathed us kids. She spent a lot of time yelling at the whole class. She didn’t use corporal punishment (that was largely a non-issue by then, but also we were as tall as she was by then).
High school: a mixture of lay (male and female) and religious. One lay teacher (mail) kind of went off the deep end and ranted at us for quite a while; another (priest) got into an argument with a classmate once; she tried to go down the hall to the principal’s office and he dragged her back in by her hair. The others were OK to very good. It may have helped that I was in the honors / college-bound courses, where the nuns tended to be those from a specific order, very well educated and very well qualified. I actually kept in touch with some of them for several years after school.
Answering another poster: Some orders are indeed devoted to teaching. Grade School was the “School Sisters of Notre Dame”. High school was Sisters of the Immaculate Heart of Mary, at least in my classes - there were other orders represeted as well.
Interestingly, I had a nun for one college course - at a major public university. She was doing a post-doc or something, and was assisting in one of my physics classes. She was SSND like my grade school teachers. I don’t believe she wore a habit or veil, not even the toned-down ones the nuns went to in the late 1960s.