No, if you happen to join a university team and decide that you no longer wish to compete for them you are free to leave to focus on your studies. There is no obligation to compete for a full season or future seasons if you happen to change your mind.
The only circumstance where this may change is if you have accepted a sport scholarship for your selected university and signed a form that states you will represent them in specific competition. Any form of sport scholarship offer would typically be renewed per academic year by the sport department. If your desire to compete was to change, this is something you could inform the staff at your selected university.