At some secondary schools, fewer than one in 10 pupils now pay to eat in their school canteen, despite efforts to improve the food on offer.

City leaders want to see junk-food exclusion zones set up around schools to stop youngsters buying greasy lunches from burger vans instead of healthy school meals.

Robert Oliver, a teacher and Sunderland Conservatives' spokesman on education, thinks the best approach is to introduce lunchtime lock-ins.

Many secondary schools on Wearside now keep at least 11-to-13-year-old pupils in school, some preventing all but sixth formers from leaving the school gates at lunchtime – and Coun Oliver wants to see all schools adopt that policy.

"My opinion is that, realistically, only lock-ins are going to make any difference," he said.

"If you look at the table of top-performing schools, they are all keeping pupils in all the time.

"Obviously I don't think that's the reason they're doing well – there are probably other reasons as well – but there's definitely a link."

Coun Oliver said he thought schools should initially introduce the policy for all Year Seven and Eight pupils and allow out older pupils, so preventing a backlash.

He said pupils would then become accustomed to eating in school and would still prefer the healthier meals to junk food as they got older.

One school that has no intention of introducing lunchtime lock-ins yet is Pennywell School.

Head teacher Kevin McDermid said he didn't think a lock-in policy would work at Pennywell, but all schools were different and others might feel it necessary.