The ASTI says the legal indemnity offered to Leaving Cert teachers "falls short" of what's required, with the union now advising members not to engage with the calculated grades system until the issue is resolved.
New guidance has been published today, which will see students reported to the Department of Education if they try to influence their mark.
It also includes an assurance that indemnity will be extended to teachers, principals and school management in case of any legal cases.
The TUI has welcomed the measure, saying "clear legal indemnity for teachers and schools [...] is of vital importance".
However, the ASTI says the indemnity falls short of what's needed.
In a statement, the union said: "[It] could potentially lead to personal liabilities for costs for second level teachers. This position is unacceptable.
"We will continue to engage with the Department of Education and Skills to secure the necessary provisions.
"In the meantime, we are advising ASTI members not to undertake any work on the process until this issue is resolved."
Education Minister Joe McHugh insists every effort has been made to protect teachers.
He said: "The least we can is to ensure they have State protection in this process, and that indemnity was agreed by Cabinet".