The children that continued taking the free supplements got two grades higher in their exams, an average of 17.7 points more, than those who did not take the pills.
However, scientists have dismissed the experiment because it did not include a second group of children taking placebo pills.
Professor Tom Sanders, head of nutrition at King's College, London said the trial was not "randomized", which is essential to avoid changes, which occur, by play of chance.
"No conclusions can be made about the benefits or otherwise. Parents who agree to supplementation of their children may differ with regard to motivate and support their children compared with those that do not," Telegraph quoted him, as saying.
"Making comparisons with a group that did not take supplements introduces selection bias. The conclusions cannot be trusted," he added.