What about something post-war? That could be interesting. Or just write about another wizarding school in another country.
I don't think parents generally pull kids out for "not being ready." I dunno, I feel they're not as... coddled in the wizarding world.
As for avoiding Mary Sues, you definitely can't break rules, and it's also important that she doesn't usurp the universe by taking over the roles of canon characters. That said, write what you want to write. It's how you write it. The exchange thing is tricky because while that's implied to have happened, it doesn't seem common so it might deter readers. But if you think you can make it work while acknowledging how uncommon it is, go for it. (Hey, what if the OC IS Charlie's Brazillian transfer friend?)
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I don't think parents generally pull kids out for "not being ready." I dunno, I feel they're not as... coddled in the wizarding world.
As for avoiding Mary Sues, you definitely can't break rules, and it's also important that she doesn't usurp the universe by taking over the roles of canon characters. That said, write what you want to write. It's how you write it. The exchange thing is tricky because while that's implied to have happened, it doesn't seem common so it might deter readers. But if you think you can make it work while acknowledging how uncommon it is, go for it. (Hey, what if the OC IS Charlie's Brazillian transfer friend?)