Recruitment and retention problems means schools 'creaking at the seams', according to the ASTI.
It's after new figures obtained under the Freedom of Information Act found nearly 14,000 people who weren't registered with the Teaching Council were hired as substitutes.
These could include student teachers along with non-qualified subject experts.
ASTI general secretary, Kieran Christie, says it still highlights issues around recruitment:
"I would imagine that the bulk of them were student teachers, and many of them would have been retired teachers, perhaps who would allow the registration to lapse. You control a system on a wing and a prayer like that. The Minister for Education needs to take some very bold and assertive moves to address this deterioration crisis."
