I think the OP is mostly correct; in the 60s, when all the cool cars were being built, not many
teenagers could afford to go buy new GTOs, GTXs, 396 Chevelles etc. Oh, some did, but not most. By the mid 70s or so though, these cars were just used cars and nearing the bottom of their depreciation cycle, so many kids could afford one as their first, or if not first, second, car whilst still in high school. Granted, the majority of teenagers cars were parental hand me downs or just plain beater junk, but during this time period, there were a significant percentage of cool cars in your average high school parking lot. For instance...
here is a photo taken on a random grey, overcast day at the high school where I was interned - small sorta suburb in the middle of nowhere middle of the country - late fall 76.
As you can see, most of the rolling stock was fairly uninteresting. However, even a cursory glance reveals 4 Mustangs, one having the Ram Air hood, a Shelby Mustang - cant tell if its a GT350 or 500 - a Cobra jet Torino, 3 or 4 Camaros, couple of neat Chevelles, couple of SS Impalas, a Cutlass or two, a Duster and even a 68 or 9 Charger way back there. Don't forget that Comet GT with a 302 either. This was just the 'main' parking lot.
The FFA guys parked over by the Ag 'barn' and that lot was mostly pickups, Blazers/Broncos and the like. Most had a rifle rack in the back window and most of those were 'occupied' ; funny, no shootings at school and no one ever got their gun stolen... But I digress. The motorcycles had their own place by the 'smoke hole' and while most of the bikes were small, 2 stroke 'dual sport' bikes, there were a couple of Z1 Kawasakis, and H2 or two, an XS650 Yamaha and an old Triumph when I was there.
What is more interesting tho are the cars NOT in this photo. Most of the more serious car guys refused to park in the lot and we occupied a neighbourhood a block or two away. On any given day at the time of this pic, the curbing in that addition saw the tyres of my 70 RoadRunner, Larrys 71 340 Duster, Johnnys 40 Ford- then SS Camaro - a 65 Impala SS, a 69 Corvette convertible, a mild custom E100, a badfast 55 Chevy, a 56 Chevy that the fellow still owns, a 69 RS/SS Camaro and two Challengers, one a 71 340 car and the other his sisters 318 unit. This was not a 'wealthy' little town either, just an average semi rural burgh that on average was probably towards the 'lower end' of what constituted 'middle class' at the time. Sure, there were a couple of rich folk and plenty of poor ones, but on average, just a middle class place deep in the Flyover Zone and that was what we drove as teenagers.