My son won't talk at school?

My son is 6 years old and in kindergarten. He attended speech therapy every tuesday for 30 minutes. She said he's doing better exchanging conversation. This week, we've been called to school to have conference with his teacher and counselor about him not taking to his clmates and teacher, At recess, he plays himself. One time the teacher saw a girl pulling his hair and he didn't even bother to sayto stop it to that girl. At home he follows directions and talk to us too. He plays with his brother, as well. He is in a gifted cl and the teacher is bothered because he is not talking. He easily gets upsets when he is pressured at school. Any advice would be greatly appreciated.