In a highly embarrassing move, the teachers' conciliation and arbitration council has found the ASTI to be in breach of the terms of the national pay deal, Sustaining Progress.
Last year the ASTI issued a directive to its members informing them that if another teacher in the school took their class away on legitimate school business, they should not allow themselves to be redeployed to teach another class without pay.
However, in a confidential judgment issued this week, the council says that the ASTI should have first exhausted all the mechanisms contained in Sustaining Progress for resolving disputes before issuing the directive.
In its judgment, the council says the directive was issued "without invoking any available industrial relations machinery which might have resolved the dispute without the necessity of issuing such a directive".
This constituted, it says, a failure by the ASTI to use the dispute resolution procedures available, and a failure to adopt a "flexible approach" to working practices as required by Sustaining Progress.
"The board, therefore, takes the view that the directive should not have been issued and certainly should not have been maintained in being once it became clear that there was a real dispute as to how the scheme for supervision/substitution should operate in particular circumstances."
In an embarrassing snub to the union, which represents more than 17,000 teachers, this means it may have to withdraw its directive and engage in conciliation and arbitration before considering reissuing the directive.
However, at a meeting of the council this month, it is understood that on the issue of whether redeployment to another class is voluntary the ASTI, along with the TUI and the INTO, succeeded in arguing that this was the case.
The Department had previously argued at the meeting, chaired by the public services arbitrator, Mr Gerard Durkan SC, that school principals could redeploy any such teachers without having to pay that teacher.
The new situation means that the Department of Education could face a substitution and supervision bill which is higher than it had previously estimated.
However, it was unclear last night exactly how much this was likely to cost, with some suggesting that other issues, such as teacher sick leave, were likely to have a bigger impact on the final cost.