It's not. It takes place in the canon of the other films. And yes, this is really stupid and this means the X-Men movies now have enough continuity errors to fill a book.
Actually, IIRC, the director of First Class did X-men 1 & 2, but not 3 or Wolverine and said in an interview that he is disregarding everything that happened in 3 and Wolverine.
So it's a half-reboot?
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