Yeah everyday is different, but the thing no one prepares you for is the legitimate amount of babysitting you do. I had honor's freshmen and basic seniors and they both had their strong and weak points. Personally I love English. I got my degree in pretty much grammar, so when I would teach about syntax and semantics and all that shit, I would get so excited. They would not. I was unprepared for the defeating feeling of 30 kids who didn't give a shit about what I was saying regardless of how interesting I tried to make it. The only times I had remote success getting through was when I would break it down into real life applications. For instance, they had to write me cover letters. A real skill that I could use to strike fear into the depths of their soul by telling them they wouldn't get a job without it.
That being said, if you can just swallow that fact down, there are always two or three of them who are genuinely interested and love what you're teaching. It's pretty amazing when they actually do learn what you're trying to teach. Also, outside of them being bored, you can have a really fun time in the classroom with high schoolers. Especially if you stay immature.
So to sum it up, I quit because I walked into a world I wasn't prepared for. I'm going back because I miss the feeling of not knowing if I'm prepared. Plus, fuck cubicles.