Figures published in parliament show that across the four state-run young offender institutions - Cookham Wood YOI in Kent, Feltham YOI in west London, Werrington YOI in Staffordshire, and Wetherby YOI in Yorkshire - young people spent an average of 13.96 hours in the classroom in 2017 - nearly two hours less than in 2015 when the figure was 15.85 hours.
The drop in classroom time comes despite a government focus in recent years on placing education at the centre of youth custody.
In December 2014 the Ministry of Justice announced that new education contracts across the four YOIs, worth a combined total of £12m a year, would see education provision extended from 15 hours to around 30 hours each week.
The government intends to pilot two "secure schools", which will provide a greater focus on education and rehabilitation, on the back of recommendations made in Charlie Taylor's review of the youth justice system.
Last year youth justice minister Philip Lee said the pilots could be up and running by the end of 2019, with the initiative rolled out nationally within 10 years.
The government said it is committed to reforming youth custody in order to improve outcomes for children and young people, and is expanding frontline staff capacity in public-sector YOIs by 20 per cent - the equivalent of 123 new recruits - and is also introducing a new youth justice specialist role.
Concerns have previously been raised by the Prisons Inspectorate that a lack of staff at Cookham Wood YOI had resulted in young people being locked in cells while "skilled and enthusiastic" professionals wait for them in empty rooms.