Why a teacher saying I was doing good but then go behind my back and say I was doing terrible?

Than another student called me stupid when she said that then another teacher said that what I say every time she leaves the room.
But everyone else in the group is fine but I'm the only one that gets treated differently.

Why people says these about me and so quick to call me stupid when I make a mistake.
Why a teacher saying I was doing good but then go behind my back and say I was doing terrible?
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