According to a study released by the education ministry on Sunday, at age 13 one student out of five is not able to solve an elementary mathematical problem.
Results in maths have become worse over the last six years in French middle schools.
Originally Vallaud-Belkacem proposed the removal of Latin and Greek languages and bilingual classes in state-run middle schools, arguing that only 20 per cent of students took these subjects and they favoured elitism.
She proposed filling the gap with interdisciplinary practical lessons.
But there has been a deluge of criticism from opposition politicians, teachers, intellectuals and even some Socialist ministers, who claimed the reform is risky and will confine excellence to private schools.
Discord was such that President François Hollande last week called on his ministers to support Vallaud-Belkacem.
French Prime Minister Manuel Valls reiterated the call on Sunday, writing in an op-ed piece in left-leaning daily Libération that the reform aims to establish more “equality” in the school system.
Former Socialist prime minister Jean-Marc Ayrault, a German teacher by profession, has even supported the call by both left-wing and right-wing teachers' unions for a strike and has asked for a partial redrafting of the reform package.
Another Socialist, former education minister Jack Lang, last week opposed the abolition of European languages and bilingual classes, which he established in 1992.
“Twenty per cent of students opt for these classes. They're a great success. Why should we stop these classes?” Lang asked the BFM TV channel. "They must be saved. Streams of excellence are not reserved for the elite, they are also for deprived areas."
Vallaud-Belkacem has stuck to her position.
“The reform will go ahead at the beginning of 2016,” she said, although she has asked a top education committee to review the reform to make it more accessible.
According to a poll published on Monday, 61 per cent of the French people are opposed to the reform.