Teaching is a bizarre, strange little world.
On one hand, you are the smartest person in the room, who tells people what to do. On the other hand, administrators were teachers, so they are used to being the smartest person in the room telling people what to do…
The kids are making things up as they go and hope you don’t notice it. You are making things up as you go and hope the kids don’t notice. (Add lines for admins and parents, too, while we’re at it.)
You are a professional with college education (usually at the masters level or better) who is continuously improving your education with extra classes, and who is told exactly when your lunch break will be, and for how long. You will be told when your break time is, and for how long. You will be told where to report for additional duties. On the other hand, you will likely not be told how to do your job, just to do it.
You will be constantly amazed by how the kids can learn the scams and get away with doing as little as possible. Same thing with some of your colleagues.
You will become an expert on your subject area, your students’ likes, dislikes, tipping points, grading software, office politics, and time-management. On the other hand, you may never know just what is in the school lunches. (The exact contents of our “chicken terriaky [sic] strips” is something I hope I never know.)
During the school year, you will be busier than many of the people you know with jobs that require them to be at work for eight hours. (Technically, I only have to be at work for 7.) During the summer, you’d better find a second job or you’ll blow through your money like you wouldn’t believe. (Personal experience, here)
You may find yourself comparing unions to lawyers. The comparison is apt: you don’t know what they’re there for until you need them. (Union officer, here)
There will be some kids that drive you to drink. Some parents, some admins, and some colleagues, too.
If you are really, really lucky, though, you can get some kids to come back after they leave to tell you how much your class (or just talking to them) meant, you might get some parents to tell you how their kid talks about your class, you might get some colleagues who are fun to hang out with, and, if you are exceptionally lucky, you might get some admins who respect you and the job you do and vice versa.
That last bit is what keeps me in teaching.