I went to school in the 90s-aughts in a school district where the median household income was ~15% below the national average. The rationale I always heard for the 7:45am high school start time was that some teenagers needed to work after school jobs to help support their families, and if school ended too late they would just drop out.

I know live in a wealthy school district (with very good athletic programs) and high school starts at 9am.

In both places at least some of the athletic/band programs started before school.

Just an anecdote though, I never heard that rationale from an official source.