A boot camp is a military training establishment with very harsh discipline.
The boot camp class mentioned in your post is clearly a pale imitation of the above, where people in a locality meet to get fit in a 'fun and social' environment.
She 'cheated' by finding a way to do sit-ups which was less strenuous than the orthodox method:
(Wikipedia)... let us see how you do a proper sit up. Firstly, lie on your back on the floor. Next, bend your knees in a 90 degrees angle and place your feet flat on the ground. Raise your arms straight and keep them parallel to the ground and pointing towards your feet. While your feet and butt are resting on the floor, slowly raise your torso off the ground and towards the thighs. You can come up as far as you can. Then go reverse through all the motions and fall back again, on the back. Do the whole thing as slowly as you can, without any jerky movements. This constitutes one sit up!
Student or Learner