Because the "opt out" would include a "workout". Choices were 1. "Do the challenge" or 2. "Do a workout". The obvious inference is that to avoid the "workout" you have to "do the challenge". That is "applying pressure". Afterwards, many kids sought medical treatment.
*** For reference: 6th graders are not routinely capable of rendering adult judgement as to what is in their best interest. The point that the "challenge was optional" is utterly irrelevant. The kids were under the care and influence of an adult caregiver.