I've been a substitute teacher off and on since 2008, but I can always learn something new!
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I'll go first! It's very important to be flexible. Working in education is a different roller coaster everyday! You might be signed up to sub for one thing and it change before you get to the building!
Well hopefully the teacher who you are subbing for has left some info for you about the class. I think you need to have good relationships with students in order for them to be able to learn anything from you, which is hard when you haven't met them before. You need to know a little about the way the class functions, what works for them. This means being flexible and open to different teaching/learning styles.