I'm sure you're like me and went to a large high school. For us it wouldn't make sense to know and remember all of our teachers. I think that their school was a small private school though so they probably had a lot more interaction with those teachers and knew them better. Plus, I think they interacted with those teachers over several years. It'd be like a small town school where K-12 is on one campus.
As for the Erica thing, I read that it was the network telling them to change one of the siblings to a girl and that's how Eric became Erica.