Kuppet on Tuesday joined teachers marking this year’s KCSE exams in asking for better pay and conducive working conditions.
Secretary-General Akelo Misori said teachers were receiving very little pay despite spending nights in congested rooms.
He gave Education Cabinet Secretary George Magoha seven days to improve the conditions, failing which he said the union would ask the teachers to withdraw their services.
Mr Misori condemned the Kenya National Examinations Council (Knec) for threatening to sack examiners who boycotted the exercise in a protest for better pay.
“We hereby urge the council to drop such threats and address the examiners’ valid grievances,” he said.
The exercise began on Saturday at Machakos School but the tutors are said to have stopped marking scripts, demanding that the pay be increased from Sh46 per script to Sh68.
The Kenya Union of Post-Primary Education Teachers wants the examiners paid a standard fee for all papers.
Mr Misori said: "We demand that the pay be [increased] to Sh100 per script."
Some of the examiners said they had been forced to sleep on the floor.
Kuppet took photos of some examiners sleeping on the floor and others sharing beds.
Mr Misori cited exposure to health risks due to overcrowding in toilets and bathrooms.
He said: “Machakos and Starehe Girls, where the business studies papers are being marked, have grossly insufficient facilities for hosting the high number of examiners - more than 1,000 per paper."