The decision by ASTI to widen its picketing today as part of its one-day strike has drawn an angry response from another teaching union.
It is understood that the Teachers' Union of Ireland (TUI) has objected to the plan to picket secondary schools with fewer than 10 ASTI members.
ASTI's action means that for the first time about 60 comprehensive, community schools and community colleges may have to close - even where TUI members greatly outnumber ASTI members.
With TUI members unwilling to pass official ASTI pickets, these schools are set to close today. Several TUI members are reported to be furious about the extension of picketing. One TUI source said last night: "This is the kind of thing that gives trade unions a bad name." Others in the TUI believe the union must show some solidarity with the ASTI.
The decision to widen the ASTI picketing was taken by its 22-member standing committee a fortnight ago - but the union has been split on the issue. Until now, ASTI's one-day strikes have been confined to about 600 schools where there are more than 10 ASTI members.
In a separate development, ASTI is now confident members will be paid for seven school days in which pupils were denied lessons this term.
The Department of Education has said the money will be docked on December 21st next but the forms necessary to process the deductions have still to reach schools.