I was a substitute teacher for a year, and lived to tell the tale.
I started out in my home district, in Florida. I taught only middle school and elementary school after my first high school assignment, when I was asked for my hall pass no fewer than six times in a day at a high school (I was only 25 - I must have looked young enough to still be a student). Most of the time, it wasn't all bad - I loved planning periods, when I could just read a book in a quiet classroom. Teachers were required to give you lesson plans (usually some worksheets or to finish up on something they had already started).
There were a few incidents, that made me start to rethink the whole 'getting a teacher's certificate' idea. The kindergarten class I had was one.
This was a special school - all free lunch students, so they had breakfast delivered to the classroom. There was, I found out later, a teacher's aide for the class I had, but they reassigned her when they found out that I was going to be a sub - a trick I learned was both illegal and common at this school. Thirty kids - five and six years old - all day (except for lunch). Half were not completely potty trained. Two had fathers in prison, and would tell you all about mama's boyfriend and what daddy was going to do when he got out of 'the big house'. One had been out the previous day and wanted to tell everyone why there was blood on his shirt, and whose it was, and what it looked like when the guy stabbed mama's boyfriend and how cool it was to stay overnight at the hospital.
At the end of the day, I crossed elementary education off my list of possible jobs.
I spent 90% of my time in middle schools. I loved it, actually - great age group, and I did a lot of gifted/arts/humanities classes, so we were on subjects I liked. I spent a week as a band director, while the real band director was conducting a massive fund raiser involving cookie dough (the band room was an icy 68 degrees all week while the boxes were allowed to defrost). I spent a month at one school, right before Christmas, and did a little of everything, including explaining the periodic table to a classroom full of geeky 6th graders who LOVED to talk about science. I subbed for a teacher at my old high school, and ran into one of my favorite teachers in the lunch room. I left Florida on a high.
I moved to Arkansas, and started again after Christmas break. Spent some time at more gifted and talented middle schools, and loved those classes. I had a science class where the teacher let me run experiments (they were doing basic stuff - water displacement, inertia - that was fun and involved a lot of creative play). I did a week as a choral director at an arts high school, and listened to chamber music all day.
I spent one heartbreaking day at an preschool/high school (the younger students were the kids of the high schoolers). I had to use my 'mandatory reporter' status once, when a nine year old girl offered to give me a lap dance after class and then did her 'sexy dance' which was all but pole dancing. I had to find a way to get her out of the class before I threw up at what would make her think that was acceptable behavior, and whereshe would have learned that.
I spent a week at a school where the teachers were cautioned not to wear red, blue or black to school, as they were gang colors and I could be shot for the wrong choice in the parking lot. I was threatened daily by students, including one six foot tall, 350 pound angry boy who came across a desk and almost knocked me out when I told him he couldn't go to the bathroom for the second time in thirty minutes, because it was obvious he was just looking to troll the halls. I was saved by a large desk and two fast-handed friends of said student, but spent half the afternoon writing up the incident. I stopped subbing at high schools altogether after that.
I went to an almost-equally tough middle school for my last couple of weeks of subbing before I got a full time office job. I did have a delightful thing happen, though - I stopped being afraid! I really enjoyed those last few days. I turned classes around behavior-wise in just a few minutes. I got an entire grade to behave in my room after I put the pushiest girl in the grade in her place: I called her by name after reading it on the paper she turned in. She was shocked that I knew who she was, and when she asked how I knew her name, I just smiled like the Mona Lisa and said "Magic." It took her five minutes to figure out it was because she'd turned in her work - and the effect stayed with the class for the rest of the week. I coasted out of there, feeling much better about myself and teaching in general.
Just don't EVER ask me to do it again!