Is it possible to pcs en route to airborne school before the G2G program?

No not at all. Cadet Command is not an airborne command and your training as a student doesn't require airborne ops so they aren't authorized to PCS anyone there with airborne TDY Enroute. Cadets in school do go, but there is a limited number of slots spread out around all the tens of thousands of students in all the ROTC programs and military academies, and Cadet Command has its own internal OML on who they choose to send

Big Army isn't sending anytime to Airborne School unless they are PCSing to an Airborne assignment or are currently in one. I pretty sure you'll have to wait for your Junior or Senior summer camp to get a slot (for some reason cadets get slots for Airborne and Air Assault without an assignment).