The start times range from 7:30a to 9:30a with departures ranging from 1:55p to 4:35p, depending on the grade level and the school.
http://www1.pgcps.org/transportation/index.aspx?id=178172
The middle school day is longer than other grade levels, and some middle schools have specialty programs (like TAG at Greenbelt Middle) that mean some kids will have longer than average bus rides. PGCPS has done a good deal of route optimization to get an optimal number of buses on the road at the same time, but given the size of the system, staggered start times seem to be necessary.
It can be difficult, but kids may have to use time on the bus or in the morning to get some of the homework and studying done, instead of trying to do it all at home in the evening.