Dunedin police say they have a strong lead in the case of suspected arson at Logan Park High School.

Fire crews were called to the school about 9am yesterday, after a toilet roll dispenser was set alight inside a girls' toilet.

The fire spread up a wall and caused significant damage, expected to cost thousands.

Four staff and two pupils were taken to Dunedin Hospital where they were treated for smoke inhalation.

Police are reviewing surveillance footage of pupils' movements in the hallway outside the toilet block.