Five students and a night guard were shot dead early Saturday morning at Lokichogio Mixed Secondary School in Turkana County.
Confirming the incident, Turkana County Commissioner Seif Matata said one of the attackers was a student at the school who was on suspension.
The attack, according to Mr Matata, occurred between 3:30am and 4am.
"Several students were injured. We cannot confirm the number until we liaise with the hospital," Mr Matata told journalists in Lodwar town before leaving for the school, which is about 200 kilometres from the Kenya-South Sudan border.
The student was only identified as Abraham and, according to Mr Matata, he is from the Toposa tribe of South Sudan.
"The preliminary information we have is that this attack was carried out by a student of the school in the company of other unknown people,” he said.
“There was a student who comes from South Sudan and he was suspended from school because of indiscipline."
The student is said to have issued threats to his teachers and students after he was kicked out of school.
"He was heard telling fellow students that he is going to burn the school or he will come back and avenge because of the suspension,” the county chief said.
“Now what we are told is that early this morning he came with about three other people and killed the watchman. (They) proceeded to the boys' dormitory where they targeted particular students whom they thought were behind his suspension."
The administrator said the gunmen shot the targeted students before moving to the girls' dormitory where they injured several others.
"So we are appealing to everybody to remain calm while the police do the investigation and apprehend the culprits," said Mr Matata.