You cannot work in the US if you enter the country on a student visa. The schools that offer instructor work can only do it for overseas students if they sponsor them for a work permit. This will accomplish what you want, so the fact you'll have two different visas is not really important. Your student visa will be revoked if you finish training earlier than you were projected to, as the school will have to report that you are no longer training with them. TSA and Homeland Security are very strict and monitor overseas students closely, especially those who come for flight training!