Thirteen to 14-year-olds were less anxious during lockdown than they had been last October, according to the University of Bristol survey.
Researchers surveyed 1,000 secondary school children in south west England.
They said the results were a “big surprise” and it raised questions about the impact of the school environment on teenagers’ mental health.
The findings come after Prof Chris Whitty, the UK’s chief medical adviser, said children were more likely to be harmed by not returning to school than they were if they caught coronavirus.
The UK’s four chief medical officers have sought to allay parents’ concerns ahead of schools reopening in England, Wales and Northern Ireland in the coming days. Schools in Scotland have already returned.
In a joint statement, the nations’ four chief medical officers said evidence suggested schools were “probably not a common route of transmission”.
And in a bid to encourage parents to send children back to school, Boris Johnson has said it is “vitally important” pupils return to the classroom, with the life chances of a generation at stake.