I generally see excessive fuel consumption as running the engine too rich. Having trained a lot of people with the stock Nexstar I would say that a properly tuned engine should give about 13-15 minutes flight time with a 2-3 minute reserve of fuel. You could have a leak somewhere but I think I would lean the engine out a little and see how it goes. In looking at my notes I see the fuel tank is 270cc, about 9.1oz. . At about 1/2 oz per minute you should get more time. Are you sure the instructor still has the stock fuel tank? He may have forgotten he installed a larger one to give more run time for trainees. Good Luck in learning to fly.