When do i unlock the pilot school in gta san andreas ?

Keep doing the storyline missions, until you have a mission called "Learning to Fly" (The 68th mission) in the desert, next to Las Venturas - this is the pilot school. After completing the mission, you'll have a pilot license - now you'll can get into the airport. Now for your second question - where is the entrance for the airport - you just have to look for it around the fence until you find it. Remember you can't enter the airport if you don't have the license, though - but if you want to fly as a passanger, search for a yellow marker and buy your ticket! Hope this information helped you!

just keep completing the main missions and eventually you unlock it. I don't remember exactly when though sorry. but to get into the airport just drive around the fence until you reach the gate. unfortunately the gate only opens if you have your pilots license

THERE IS A MISSION THAT U HAVE TO COMPLETE AND ITS LOCATED IN THE DESERT ABOVE SAN FIERRO SO U BASICLY HAVE TO BE ABLE TO GET TO LAS VENTUTAS WITHOUT GETTING THE WANTED LEVELS