Hmm, I may be in the minority here, but I think A Level teachers should be able to teach the full course (Pure/Mech/Stats) There's too much overlap between the topics, and it puts students at a disadvantage if their teachers are saying "sorry, I can't do that bit" when it comes to revision/support.
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I wouldn't want to share an A Level class, too difficult to ensure expectations are the same and also that language used is the same. You can build relationships and a classroom culture if it's "your class".
I wonder if it's what you get used to, I would certainly find it strange to suddenly be asked to share a class.