How can I learn air traffic control as a high-schooler?
Join the Vatsim network.  This is a network of simulated pilots flying X-Plane, FSX, etc., and simulated controllers.  The simulated controllers use, as much as they can in a hobbyist network, real phraseology and procedures.  I personally knew several real-world controllers who controlled on the Vatsim network in their off-time, and I know several hobbyist controllers who went on to become real air traffic controllers. 

While controlling on a hobby network is not the same as controlling real airplanes, you won't learn a lot you'll have to relearn.  That's because the hobbyist controllers on Vatsim make a serious effort to follow real procedures.  When they discuss air traffic control with each other, you frequently see references to the ".65", the official bible of ATC.