Two new modular classrooms will be built for the Grange Academy campus in Kilmarnock to cope with rising pupil numbers.
The decision was made after reports of overcrowding in corridors and Annanhill Primary pupils feeling unsafe using secondary school toilets.
It is hoped they will be open by the end of the year and councillors approved the plan at an East Ayrshire Council cabinet meeting on Wednesday.
Speaking at the meeting, council leader Douglas Reid (SNP) said: "This will be welcomed by all three schools.
"We are aware the pressure is on to find a solution. Hopefully we can do work over the summer and get this implemented as soon as we can."
Conservative Group leader Tom Cook said education boss Linda McAulay-Griffiths and her team had been “performing somersaults” to fit children in. The Kilmarnock West and Crosshouse member described it as a “semi-permanent” expansion to the school.
Andrew Kennedy, the council's head of facilities and property management, said the classrooms "will meet full building standards for permanent facilities."
Primary children have been taught in four Grange Academy classrooms because of high demand. The overall Grange campus, which is also home to Park School, has exceeded capacity by 93 pupils.
Annanhill has room for 459 pupils but the school has been teaching 526. For August, the council received 31 placing requests for the primary.
Councillors are due to receive an update later on the finals costs of the new classrooms, progress on work and longer term needs for the campus.
The meeting heard there would be "obstacles" to overcome as Grange is a Public Private Partnership (PPP) school.
A council report said: "It has become apparent that additional accommodation will be required in the short to medium term and to facilitate this it is proposed to take forward an option to install a separate two class modular unit in the campus grounds.
"Recognising this will not be available for the start of the new term, as early progress would be beneficial, it is therefore proposed to progress works with the PPP provider over the summer to facilitate confirmation of final costs and time scales for as early an installation as possible."
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