On the first day of school, you got to be real careful where you sit. You walk into the classroom and just plunk your stuff down on any old desk, and the next thing you know the teacher is saying, 'I hope you all like where you're sitting, because these are your permanent seats.'

This quote is telling us that on the first day of school, you should be careful where you sit. If you go into the classroom and choose a seat without thinking about it, the teacher may tell you that seat will be yours for the whole school year. This means you won’t be able to change it, so you should make sure you choose a seat that you like.