Last year 17,000 16-year-olds - about 3 per cent of the age group - left school at Easter, instead of waiting until June.
Ms Morris said: "Many of those who left early left without any formal qualifications to show for their years at school. These young people are those most at risk of becoming disaffected, disenchanted and left behind."
The school-leaving age was raised to 15 50 years ago. Then in 1972 it was raised again to the current level.
The ruling comes into effect immediately. Advertisements will be placed in national newspapers to inform parents and pupils.