At least two classrooms were badly damaged in a fire at Sacred Heart Girl's College in Hamilton overnight.
A police spokeswoman confirmed this morning the fire is being treated as suspicious. Detectives will be at the scene today.
The fire began just before midnight and a number of people saw it and called 111.
It took 40 firefighters five hours to put the flames out.
Fire and Emergency shift manager Paul Redden said the top floor of a two-storey block was well alight when trucks arrived.
A fire investigator is also at the school this morning.