Dozens of schools are coming under pressure to stop allowing pupils home early on the last day of term, amid concerns about child welfare.
Heads in Cambridgeshire have been told not to let pupils leave before the end of the normal day because they remain responsible for their safety.
And groups of older pupils have been returning to their primary schools - which do not finish early - and disrupting lessons.
Chris Keates, general secretary of the NASUWT teachers' union, said some schools used an earlier finish to maintain discipline.
"Young people can get quite excited about breaks and schools have used different finishing times as a management strategy to ensure good behaviour," she said.
"It has to be a matter of judgment for individual schools. They are mindful of the safety of young people and when schools close should be down to their discretion."
Local union representatives have criticized the move by Cambridgeshire County Council, saying it could create resentment among staff.
Geoff Fewtrell, county secretary of the NASUWT, said: "Most secondary schools finish at lunchtime and the staff have a get-together. It's good for morale.
"We see industry doing it, and this has been going on for years. It's a bit of a dead time anyway."
Ben Slade, principal at Manor Community College, said: "Some students might not have anywhere to go, and our responsibility for them lasts until the end of the school day."