Seven students were seriously injured during a fight at Jamhuri High School on Tuesday night.
Some students admitted to hospitals have stab wounds.
It is not clear how some students managed to get into the school with machetes and knives.
The fight broke out after a section of the students protested alleged discrimination on the basis of religion, students say.
The school's principal, Fred Awuor, was also injured during the brawl.
On Wednesday morning, Nairobi police bosses visited the school to investigate the incident.