This is disagreeing signal. If you are the teacher and you see your students doing this then what you need to do is get them out of this position: ice breakers help to make people warm up and be a little more receptive or you could give the pupil a pen or something to hold (they now have to break this position and lean forward and unfold. Note, negotiators use this technique.
This show defensiveness AND hostility. The aggressiveness is more marked when clustered with clenched teeth and a red face.
This shows a negative, restrained attitude. People are often seen doing this (a form of self hugging) when they are anxious e.g. a dentist's waiting room or a court room.
Click here to find out more about your arms.