Neil Morrison, head of Portlethen Academy in Aberdeenshire, fears diet may be one of the factors causing pupils to have below-average grades.
The school's SQA grades for 2013 were lower than those achieved at nearby Mackie Academy and Mearns Academy.
Now the canteen has been opening earlier to serve pupils breakfasts to help set them up for the day.
Mr Morrison spoke of his concerns at a meeting where councillors were considering a report on school performance. He said Portlethen Academy staff had concerns about statistics compared to other schools.
Mr Morrison said: "We have worries about our girls as they are not achieving results as high as we would like. We have done some interesting work around diet. It is quite worrying that a number of our young girls do not have breakfast."
SQA results for Kincardine and Mearns secondary schools were set out in a report discussed by councillors. The report put forward to the Kincardine and Mearns area committee stated that S6 pupils at Portlethen Academy obtained lower results. Female pupils at Mearns Academy in Laurencekirk and Mackie Academy in Stonehaven tended to perform better in their exams than males.
Mr Morrison said staff were looking to see if there were any other ways they could help improve grades.