Is it correct to say sitting in a chair or should it technically be sitting on a chair? Can one sit in a solid object like a chair? I understand sitting in a house or box or car.
You would sit in an armchair, where the body is partly enclosed by the arms of the chair and the broad, high back. We even talk about 'sinking into a comfortable armchair and putting my feet up."
With a dining room chair, or any flat-base chair (the kind of chair with a flat sheet of wood for sitting on), with either no arms, or just wooden arm rests; and an open back rest, then we sit 'on' sit a chair, just as we would sit on a stool.
In a classroom situation, we would say, "Sit in your seat."