Why do teachers keep you behind?

I understand some teachers like students to get better grades but my friend gets good grades. We work as a 3 in science and every lesson we get kept behind for no reason even if we finish work. The teacher doesn't tell others to stay behind, doesn't check their work, doesn't ask them questions. The 3 of us arnt the only people with colored skin there's another girl but for some reasons it's just always the 3 of us and its making me curious on why this teacher is repetitively keeping us behind
Why do teachers keep you behind?
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