A road was shut off by police after officers arrived at the scene of a break in to find a strong smell of gas.
Officers were called to the former Sunderland High School, off Mowbray Road, at 9.50am after a report the building had been broken into.
Gray Road, which borders the mothballed school site, was sealed off between Ryhope Road and near to its junction with Mowbray Close.
The cordon has since been lifted.
Inquiries into the burglary are ongoing.
The school, which had several sites, closed in July last year due to a fall in pupil numbers.
Anyone with information is asked to contact Northumbria Police on 101 or Crimestoppers anonymously on 0800 555 111.