When I started teaching, in 1975, the school day finished at 3.30pm.
However it was decided that the lunch break was too long, because it resulted in lots of bored teenagers roaming the streets at lunchtime. Given that there were two other secondary schools within easy walking distance, that meant that gang rivalries were likely to break out. So the lunch break was cut from 70 minutes to 50 minutes. The start of the school day was also moved ten minutes earlier, meaning that the school day could end at 3pm without the loss of any teaching time.
Around the same time, other Sheffield schools also reviewed the hours when formal teaching took place. Many of them adopted a similar model to our own, but at least one (by starting the school day earlier) was able to finish at 2.40pm without the loss of any teaching time.
Chris