Calm down first- that was actually pretty mild. You wouldn't be the first teacher to say something like that (or worse- crap is a word I've used, and many of my colleagues think nothing off using the word p**s- otherwise called liquid excrement) and it will take a lot more than that to get you fired. You did that incidentally because you are in fact human, as much so as anyone else, and while you are expected to have much more control over yourself than other adults, you are not going to be perfect. If your administration has a problem with it, they will take it up with you- just be willing to answer honestly and justly, and if an error has been made, 'fess up, discuss options, and then do differently. I've had some moments this week that would drive the average person into long strings of expletives (and in many cases, probably physical violence). Everyone can break occasionally, when the circumstances are right. You just had a mild fracture- recover, compose, and move on.