Hundreds of students were not in stable work or education last year within 12 months of leaving school in Calderdale, new figures reveal.
The Association of School and College Leaders says it is a "huge concern” that teenagers across England are drifting into insecurity after 14 years of school education.
Department for Education statistics show that 15 per cent of the 1,954 students who finished their 16 to 18 study in 2016-17 were not in education, employment or an apprenticeship within a year
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A further seven per cent had no activity recorded.
The figures cover pupils from state-funded mainstream schools and colleges, and only those in continuous study, work or training for at least six months were included in one of the categories.
Continued education was the most popular choice for school leavers in the area, with 48% of students going to university, further education college, or courses elsewhere.
Work was next most common – 20 per cent went into employment.
And 10 per cent were in an apprenticeship for at least half of 2017-18.
The rate of students in Calderdale who were not in education, employment or training within a year of finishing school was slightly higher than that across England, where the figure stood at 13 per cent.
And 47 per cent went into education, 25 per cent started a job, and 10 per cent undertook an apprenticeship.
The activity of six per cent of students was not captured in the data.
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Julie McCulloch, director of policy at the Association of School and College Leaders, said: “It is of huge concern that one in eight 18 year olds appear to drift into a netherworld of insecurity and aimlessness after 14 years of education.
“There will be many differing situations behind the statistics, but there is no disguising the fact that too many young people are slipping through the cracks.”
She added that schools and colleges are not getting enough government funding to support young people.
Laura-Jane Rawlings, chief executive of Youth Employment UK, which works to reduce youth unemployment, said young people are three times more likely to be unemployed than any other group, and that long-term unemployment could be “scarring” for the individual.
She added: “It is crucial therefore that schools, colleges and the local authorities supporting young people are ensuring that every young person moves into a post-18 education, training or employment destination that works for them.”
A Department for Education spokeswoman said the statistics do not show “a complete picture”, as private students, those who studied abroad or people working in short-term jobs are not included.
She added: “Young people aged 16 to 18 are participating in education and training at their highest rate since consistent records began, and the overall proportion of 16 to 24-year-olds not in education, employment or training has fallen since 2010.”