Late summer practice in much of the United States happens in genuine heat. State associations set rules for it, and those rules encode compromises students never see.
The physiology is not in dispute
Heat illness develops when the body produces heat faster than it can shed it, and humidity is central because it limits evaporation, which is the main cooling route.
Acclimatization matters: a body adapts over roughly a week or two of graduated exposure, which is why the first days of a season carry higher risk than later ones at the same temperature.
Equipment loads, fitness level, illness and some medications all change how a given athlete handles conditions, which is why an athletic trainer or physician is the right person for an individual assessment rather than a general rule.
Policy translates physiology into schedules
State associations typically specify graduated equipment introduction, mandatory rest and hydration breaks, and thresholds at which practice must move indoors or stop.
Those thresholds are usually expressed with a measure combining temperature and humidity rather than temperature alone, because temperature alone understates the risk on humid days.
Rules vary by state and are revised over time, so the applicable policy is the one published by that state's association and enforced by the school, not a general national standard.
Facility and staffing constraints shape the compromise
Moving practice to early morning requires transportation and adult supervision at hours that conflict with employment, and gyms are frequently already committed to other teams.
So schedules land in the late afternoon partly because that is when buses run, coaches are free and facilities are open, which is a logistics answer to a physiological question.
Monitoring depends on who is present
A certified athletic trainer on site can measure conditions, recognize early heat illness and initiate cooling immediately, and that presence is the strongest protective factor available.
Many schools, particularly smaller and rural ones, cannot staff a trainer for every team, and coverage tends to follow the same priority order as officials and facilities.
What athletes and families can reasonably ask
Whether the school measures conditions on site rather than using a regional forecast, whether cooling equipment is available at practice, and who is trained to use it are all answerable questions.
An athlete who feels unwell in heat should stop and say so rather than finish the drill, and any suspected heat illness is an emergency for medical professionals rather than something to manage on the field.