Here is an undefined list of things I wish I knew before I started teaching and some I knew but didn't really heed and should have:
1. There is no one lesson plan format. If someone wants "lessons" from you ahead of time, you could be turning in anything from red Sharpie scrawl across a napkin to three page documents that site local/state/national standards. Don't do the latter if you can do the former. This, after three years of teaching and beating myself up about my inability to make and keep plans, is part of my actual philosophy of teaching. If I can remember a lesson that I've done before, and I want to do it again, then it was probably really good. If not, then I need to go back to the drawing board. I know this won't work and won't make sense for many people, but, for me, I keep my lessons fresh and my own excitement and engagement with the material high by doing this, and the kids respond.
2. Know when are grades due. Turn them in a day early if possible. Make your admin's life easier, and he/she should return the favor.
3. Be especially nice to custodians and secretaries. They know what's up about everything no one bothered to tell you but expects you to know: Who do I go to for substitutes/ professional leave/ field trip forms/ etc.? How do I take attendance/ sign in/ notate tardies? What secret things am I expected to do during testing and finals?
4. Don't be offended when people do not smile or say hi in the hallway. People are busy. You too will become pseudo-bi-polar. This does not count for private school. Always smile and say hi in private school. If you don't, people will think you have mental problems and are not socially well-adjusted. To them, this is icky.
5. Keep handwritten, like with a pencil or pen, records. Keep everything. Your electronic grade book will erase your grades at some point. You are not crazy (this time!). It happens. Note: I don't actually do this, but I should. But I don't. I keep wee little scraps of paper with random grades for random students scrunched up at the bottom of my big bag in case of emergency. Please pray for me.
6. Ignore co-workers who are threatened/pissed by your general presence and/or energy--you are part of the natural cycle of things and should be valued for your aforementioned energy and new, sprightly ideas. Of course, you should also value the wisdom and experience of your more "seasoned" peers. Respect should go both ways.
7. Do not do anything unlawful within school district. This includes (sigh) sleeping with students, showing up to school drunk and/or high, drinking or doing drugs with your students, dealing to or being dealt drugs by your students, having dirty pictur*s on your desktop, hitting students, performing seances or exorcisms with students, or doing any other thing you don't want your very old, fundamentalist granny to know about. This rule seems so obvious to me, yet I have seen teachers do every one of aforementioned things during the course of three years.
8. Stay away from negative, I-hate-children teachers. Avoid break-room, and eat in classroom if necessary. In private school, amend somewhat to eat lunch with others a couple times of week. This is a social thing and, therefore, necessary.
9. In public school, do not get sick or take any days off from work. No one understands that people with 103 degree fevers are not supposed to be teaching, and boss-people will think you are unreliable and/or faking. I have not found this to be the case in private school.
10. Do not hook-up with co-workers. It is really really gross. If you do, keep your mouth sealed with Krazy Glue ('cause you are Krazy!). Your co-workers do not want to know about your nasty social life. Tee hee.
11. Do not let your students be your "friend" on myspace or facebook. Bad idea (see #10--students DO want to know about your nasty social life, and their memories last far longer than the brainiest elephant). Do like your "friend" hipteacher and create a facebook for you as a teacher. Kids will think you are rad and will like having you wish them "Happy Birthday, Dude!" on their wall. They will even write on your wall to ask you homework questions and stuff. This might make you feel cool. Also, this is a nice way to keep up with all your former students who have moved on to be real adults and who are doing neat stuff you wanna know about.
12. Oh, yes. The magic three: eat, pee, sleep. Those are the hardest of all.