Calvin's acquiescence was a relief- Kurt didn't want to go round and round much longer. He was about to break into a smile, more from appreciation of the fact that Calvin finally got it than from any feelings of happiness, until Calvin opened his mouth again and ruined everything. That was an inevitability and Kurt was way too optimistic to have imagined that Calvin could just accept anything and move on. At the question his smile faltered and then disappeared entirely, replaced by a scowl.
"Professor Xavier decides, so as it stands at the moment I am the leader-" which Calvin knew very well. If the man thought he would ever lead the team he had another thing coming. He'd earned his keep but not his stripes. Before this could get any worse, Kurt disappeared in a puff of smoke. They could deal with all the logistics later, when Kurt had regained some patience.