Stalling means loss of lift.
That is caused by pulling back on the stick too much.
Also you are raising the landing gear...
Take a break from the School.
Go back to the Los Santos International Airport
Enter the field. by climbing to the roof of the Guard house.
You can then open the gate from the inside if your vehicle is not parked too close to the gate.
Steal either the Dodo, or Shamel to practice flying.
Flying the Dodo for 25 minutes or the Shamel for 15 minutes will cause a notice to appear that you have a pilots license. It is really only a Gate Pass allowing entry to the airfields, you must still complete the school.
Using this method will allow practice without time limits. The Jet has retractable landing gear the Dodo does not.
Do you have the DVD or the STEAM registered (download) Game?