Delegates voted for a ballot if the Executive's response to the interim report of the ministerial working group on staffing resources and class sizes - due later this summer - fails to match EIS policy of cutting the maximum size to 20 pupils in every class.
A bid by the more militant wing of the union to try to ensure that any industrial action would take place before the Scottish parliamentary elections next May was declared unlawful under EIS procedural rules.
It also emerged that the final report on the class sizes working group was unlikely to be published before the elections.
However, Larry Flanagan, of Glasgow, warned that the Scottish elections would be crucial to the outcome of the EIS campaign to lower class sizes.
"We have to press other political parties (on this) because we don't know who will be making the decisions after the May elections next year," he said.
Annie McCrae, of Edinburgh, predicted that the interim report would say nothing new - nor would the final report when it came out.
"The Executive's response has to give us some answers over this or we will have to take action," she said.
She added that it was important to ensure that by cutting S1 and S2 classes to 20 in English and maths, this did not create a bulge in class numbers higher up the school.
"I started this session with a Higher English class of 30. In primary, it is not enough to have P1 classes capped at 25 - they have to be 25 in P2 and P3 as well," she said.
Pam Crayk, of Edinburgh, said that her P1 class of 30 children included children with autism, Asperger's Syndrome, Down's Syndrome, ADHD and several children who did not have English as their first language.
"We have to do whatever is necessary now to secure the future education of our children," she said.