Students who collect their GCSE results in two weeks and plan to stay in full time education or unpaid training are still eligible for child benefit.
But their parents must inform HMRC of their plans or it will automatically be stopped on August 31.
Forms have been sent to more than 620,000 people whose benefit will stop if they don’t return them.
So far, only half have been sent back.
If a child is staying in full time education or unpaid training the form needs to arrive with HMRC before August 31 to ensure payments continue.
Customers who have lost their forms can, like 15 million other taxpayers, create an online Personal Tax Account www.gov.uk/personal-tax-account to make their declaration.
Angela MacDonald, Director General of Customer Services, said: “Please let us know if your child is continuing in education or training so that you receive the correct benefits without interruption.
“Leaving school and collecting exam results is a rite of passage and we want to ensure that our support remains consistent as they take that next step into further education or training, so please update us as soon as you can.”
Full time education is classed as A Levels, International Baccalaureate, NVQs and other vocational qualifications up to level three. Those taking BTEC courses, university degrees or courses funded by an employer do not qualify for child benefit.
More detailed information about the training and apprenticeships in England, Wales and Northern Ireland eligible for Child Benefit recipients is available online at www.gov.uk/child-benefit-16-19
If customers have not returned their form by August 31 they can call the Child Benefit Office on 0300 200 3100.