My store (district even) is very strict about minors. Mainly because my state has strict work permit requirements and schools will often have a minor submit a month of timecards and pull the work permits if the minor is in violation. On any weekday, minors cannot work longer than 3.75 hours exactly and must clock out before 9:30. Weekends and any day a minor is on "m waiver" status for winter breaks and such a minor is permitted to work up to exactly 7.75 hours and clock out before 9:30. My ETL-HR explained the .75 as a 15-minute buffer between 4 and 8 hours (the times that would put a work permit in state law violation) in case you're late to the timeclock. However, the HR team often audits the time cards of minors and will write you up if you're in violation of the 3.75/7.75 rule.
As far as the times of work, my store is actually rather flexible. The only limit my HR team has spoken of is that no minor can start before 4 AM (state work permit regulation.) I've started at 6, 6:30, 7, 7:30, and 8 AM before. As far as turning 18 but still being in high school, check your state law. In my state, at age 18, attendance at high school is not compulsory, therefore students are not required to get a work permit and can work with zero restrictions. Sometimes, we have had minors graduate at 17 (just had one last year even) and they don't need a work permit because they have their diploma, but they must still follow the federal laws regarding powered equipment and hazardous occupations.
