About 20 parents of children in De La Salle College in west Belfast protested outside the school on Tuesday about ongoing staff absences at the school.
About 24 of 70 teachers were absent on Monday from the Andersonstown school.
Some protesting parents yesterday reportedly complained about the disruption to GCSE and A-Level teaching.
It was reported that some 50 sixth-form students left the school on Tuesday to join the protest outside.
A source close to the school claimed that all teacher absences on Monday were covered by Department of Education approved teachers in the relevant subjects.
The school has not offered any comment to the News Letter.
It is understood there was a similar level of staff absence in November, which was followed by an agreed action plan to address issues raised.
Gerry Murphy, Northern Secretary of the Irish National Teachers’ Organisation (INTO) said he is “engaged with the school’s management, and employing authority in an attempt to address a number of ongoing issues within De La Salle College”.
He added: “INTO is increasingly concerned with the slow progress towards a resolution, and is now considering further targeted industrial action within the school.”
Sinn Fein MP for West Belfast Paul Maskey has called on the governing authority, the Catholic Council for Maintained Schools (CCMS), to strive to resolve the ongoing issues.
“The ongoing issues at De La Salle College are proving very disruptive for pupils and their education,” he said, adding that he is seeking an urgent meeting with CCMS “to press for a resolution”.
In November the BBC reported that one third of staff had called in sick after teacher concerns over how an incident was handled.
In January, a “strategic plan for renewal” was drawn up “to address a range of issues raised by staff relating to the operation of the school”.
CCMS said on Tuesday that when issues first arose a strategic group was set up, including governors, CCMS and four unions - and a strategic plan for renewal was formed.
“We can assure parents that all interests in the strategy group are working together to ensure that there is no delay in this process and that there is total commitment from all to continue to work towards a resolution,” it said.
Education Minister John O’Dowd said he was “very concerned about the ongoing issues” and urged all involved “to bring this to a resolution as soon as possible”.