Just a guess, but let's look at this from the CYA philosophy.
Your school system and/or state likely has one or more lawyers on staff. Those lawyers believe firmly in CYA. They envision a scenario similar to the following:
School bus driver with very high BMI falls asleep without warning (or suffers heart attack and/or looses conciousness from an abnormal heart rhythm...all known health conditions which can result from sleep apnea) while driving a bus-load of children to school.
Several children's parents sue school distric (and provider's who performed CDL physical) for injuries and "emotional damages".
Plantiff's legal team states "Of course, this bus driver had a very high BMI...which means she had a very high risk of daytime fatigue and adverse cardiac outcomes...which means any competent employer and any competent healthcare provider would, of course, insist on a sleep study.
Everyone involved leaves very unhappy. Except the trial lawyers.
Not saying any of the above is right, but I can see it happening.
Also, it might simply be a state requirement that any person with a BMI over X is required to get a sleep study. Request a copy of your state's CDL physical guidelines. They should spell out exactly what is required. If there are no regulations regarding sleep apnea testing in patients who are not symptomatic, then no worries.
