Fire has gutted a dormitory at Lamu Boys Secondary School.
County police commander Muchangi Kioi says there are no casualties as students were in the field having games.
Mr Kioi says the cause of the incident is not known.
Mr Masjid Basheikh, the school board chairman, said property worth two million has gone up in flames.
This is the second incident to befall the school in less than a month.
On May 10, five students were injured after a fire broke out in one of the dormitories.
Mr Basheikh said that the students were hurt in a stampede when they tried to salvage their property.
He said about 30 mattresses and beds were destroyed.
"Our students tried their best to put out the fire. It was around 8.25pm. Some mattresses, beds and students clothes and boxes were destroyed."