Children starting secondary schools in England this week will be the first to be legally required to stay in education until they are 17.
The rise in the age at which pupils can end their studies is just one of several changes taking effect in English schools this term.
New diplomas for subjects such as engineering and construction are also being phased in.
Head teachers say too many initiatives are being introduced at once.
The increase in the education leaving age is the first such change since 1972, when it was raised from 15 to 16.