In a letter seen by The TES, Jim Knight, the schools minister, said that some of the country's most vulnerable 16- and 17-year-olds would have reasonable excuses not to participate - casting doubt over his previous claim that the new Education and Skills Bill would "effectively" create 100 per cent participation.
Teenagers who could be excused include drug addicts, those with learning difficulties, temporary or long-term heath problems, and teenage mothers and young carers.
Mr Knight stressed that these groups will not be formally exempt from the law and that local authorities should be trying to provide them with an education. "An exemption could mean an easy way out of helping them and could deny them opportunities," he said.
However, he said that if some teenagers in those groups had strong reasons not to be in education - such as the fact they were undergoing treatment for drug addiction - it was crucial that they did not face enforcement action and fines.
The Princess Royal Trust for Carers had previously warned that the Bill offered nothing to 16- and 17-year-olds caring for family members except the "threat of prosecution to young people who fail to attend school, regardless of the pressures on them".
David Laws, the Liberal Democrat spokesman on children and schools, this week welcomed the letter as a step in the right direction, as he and charities have warned that young people in difficult circumstances could be penalised.
Mr Laws believes all the categories laid out in the letter seen by The TES could make up as much as 5 per cent of 16 and 17-year-olds, and he is campaigning for a new legal category of young people who need support before they are in a position to be educated.
Barnado's, the charity for children and young people, said it was crucial that safeguards were in place to ensure that the "reasonable excuse" clause was not used to exempt whole groups, rather than individuals from education and training.
It is pushing for local audits to ensure sufficient support to help young people in difficult circumstances is in place for them before the law changes.