Yavneh College has urged local synagogue members and parents of pupils to sign an online petition calling on Hertfordshire County Council to fund the proposed expansion of the Borehamwood school to include a primary section. More than 400 signatures had been received within 24 hours of the petition being launched on the council's website.
In December, Hertfordshire announced that it could not meet the £4 million needed for a primary section on the Yavneh site.
It will reconsider the issue in May and the school remains hopeful of a positive outcome, allowing the primary to open in 2013.
In a letter to supporters, governors' vice-chair Daniel Album noted the acute shortage of Jewish primary provision in the area, with considerable oversubscription at Hertsmere Jewish Primary in Radlett and Clore Shalom in Shenley.
In consequence, "more than 80 Jewish parents reluctantly had to send their children out of the area to find reception places".