You have to make it down to the Opunsezmee Roll Call Rail in under 20 seconds.
While the second schoolbell is ringing, (Timer will be around 1:41) start
grinding the rail. When you reach the bottom (and get the Opunsezmee gap), the
doors to the Gym will open up. The Gym is the set of doors after the LunchTable
gap. It shouldn't be hard to find. The Doors will stay open forever after that
point (unless you play carrier mode with a different character). The Gym has a
bunch of gaps, and easy ways to rack up points, but no cash icons or anything
required to get 100%