TfL operates a network of dedicated school bus routes — the 600-series (numbered 601 to 699) — running only on weekdays during school term-time to take secondary-school pupils to and from school.
The 600-series consists of around 100 routes designed around the start and finish times of secondary schools. They typically run two or three journeys in the morning (07:30–09:00) and two or three in the afternoon (14:30–16:30), and the rest of the day they're parked.
Schools generate huge tidal demand — hundreds of children needing to reach the same gate within a 15-minute window. Squeezing them onto regular routes would overcrowd buses and leave fare-paying passengers behind. Dedicated 600-series buses match capacity to demand.