I've got too much on my mind to teach today

Teaching is a strange job. If you feel like crap in the morning, you have several options:
1) Call the subfinder. Don't go. Send in a lesson plan and relax. The problem with this is that most substitutes are complete morons, so when you get back the next day you discover that the kids spent the pervious class period watching the NCAA tournament, not finishing their reading of A DOLL'S HOUSE. So now they're a day behind. Plus, they've probably tagged the crap out of the room and thrown trash all over the floor and probably stolen things.

2) Show a movie or assign a group activity. This is my favorite. You can contain your class and monitor their behavior while you play on the internet. The disadvantages are few - you have to have seen the movie before to offset any problems, or you have to make sure the activity is well planned out. They see right through busywork and reject it out of hand. I have a few projects line up in case of emergency, and a few movies for the same reason. They love CRASH. You just have to make sure it connects, although there are some teachers who show completely innapropriate fare like TEXAS CHAINSAW MASSACRE.

3) Put on a brave face. Sometimes this can't be avoided. The kids rebel against the group assignment, or you don't have a decent movie, or you're right in the middle of an important series of lessons and can't break off without doing damage to your momentum. You have to stand in front of the kids pretending nothing's wrong. Sometimes, if it's a really good group, you can tell them you're having a bad day and they'll cut you some slack. But sometimes they expect you to be a saint, untouchable and inhuman.

Either way, you can't just call in sick. Not when you're a teacher. Chaos would erupt and the classroom would be in flames.