New classes highlight what you forget you know

For my interview at Ellesmere College I have been asked to teach a 15 minute lesson on the rock cycle. It has been interesting exercise in planning. I don't know their prior learning. I don't know their levels of literacy and numeracy. I don't know the balance of the class, are there a few brilliant kids amongst a bunch of strugglers, are there a few strugglers in a bunch of brilliant kids.
It's highlighted how I take for granted how much I know about the kids I teach. I know which kids I can throw challenges at and they will persevere till they succeed. I know which kids will need to be scaffolds. I know which kids will need handholding. I know which kids could be any one of those but will avoid doing anything if they can help it.