The Post Primary Teachers' Association says a new computer system has plunged the Correspondence School into crisis and needs to be abandoned.
In a letter to the school's board, the association outlined 16 failures of the system which went live in March.
It says students' education has been disrupted, and the school must cut its losses and try something new.
PPTA general secretary Kevin Bunker says the failures are having a negative effect on teaching staff.
"Our members have really come to the end of their tether.
"They're entitled to have a system that is effective, that works, that works for students and for the school."
The school's Board of Trustee's chair, Trish McKelvey, declined to be interviewed on Checkpoint.
Instead, in a statement, she says the board on Wednesday held a workshop with managers across the school to consider how to make the system more workable.
And she says the project was overseen by a board which included a representative from the Ministry of Education.