The States will carry out an independent review into plans for the redevelopment of La Mare De Carteret schools.
The Education Department warned the review could mean a delay.
Even if the programme is only held up by a matter of weeks, Education believe it may have to shift the entire rebuild to the following school year.
The department had hoped to have the new school ready to take students by 2017.
But the States were concerned the current plans were too expensive, and included more capacity for students than would be needed.
Having agreed to the review, which was put forward as an amendment, deputies still need to cast a final vote approving the rebuild pending that review.
The independent review will assess if the rebuild plans offer value for money and a report will return to the States in February next year.
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