How hyper is too hyper

I have a 5 year old that just started kindergarten. He too does not stay in his seat. He is very loveable but very active. He loves to be the center of attention. He was great the first two weeks (honeymoon period) but now has gotten to the point where the teacher is frustrated. He has become part of a group of 3 that are now dominating the class with disruptive behaviours. The teacher actually recommends that I pull him out because of his "immaturity" he will be 6 in March. She states he is at the top of his class academically but his maturity is not there. She recommends he retry kindergarten next year at 6 almost 7. unfortunately his brother who has just turned 3 will be starting kindergarten next year as he is "advanced".