Only one in three secondary schools is carrying out classroom tests as part of the revamped Junior Cert.
The first assessments for next year's students are due to get underway from today, but the biggest secondary teachers' union has told members not to take part in them.
The ASTI is opposed to the changes which would see teachers marking their own students for State exams.
Thousands of students risk missing out on 10% of their English exams if the action continues.
ASTI general secretary Kieran Christie defended the action and said their objections were are based on principle, not money.
He said: "Higher level and lower-level papers are going to be abolished (in favour of) a common level paper, and that's going to be very detrimental to the interests of students.
"Our members took two days of strike action over the Junior Cycle. It wasn't in relation to pay… they stood up for the integrity of the system. This is about standards."