Teachers are calling for the summer GCSE and A-level exams timetable to take into account the impact of Ramadan on Muslim students.
Over the next few years, the holy month, which Muslims observe by fasting, will increasingly fall during the exams period, which could affect teenagers' results, the Association of Teachers and Lecturers (ATL) heard.
Barry Lingard, told the union's annual conference in Manchester that ATL had been in discussions with the Joint Council for Qualifications (JCQ) - which represents exam boards - and Muslim organisations - over the issue.
Proposing a resolution on the topic Mr Lingard, of the union's executive said it was "highly unlikely" that exam boards will change summer timetables.
But he added: "JCQ have said that they will work collaboratively on a timetable with Muslim groups to review whether a balance of morning and afternoon slots for large entry exams is more appropriate, or just morning slots."
It is thought that this could mean scheduling exams taken by large numbers of students - such as English and maths - in the morning only.
Abdul Choudhury, an ATL member from Tower Hamlets, London, said that Ramadan will have "quite an effect on a number of schools across our country" compared to the last time it fell during exams season, around 25 years ago.
"We have quite a significant number of Muslim students and I have no doubt whatsoever that standards will be affected by this," he said.
"We work so hard as teachers to try and get just that one mark, that two marks, and if kids are going into their exams - 16-year-olds, 18-year-olds, even university students - are going into exams without any water, and food, it will have an effect on their overall results."
Delegates passed the resolution, which called on the union to raise awareness of the potential impact of Ramadan on Muslim students' education.
JCQ director Michael Turner said: "We are consulting on the exams timetable for 2015 and we will be looking at the impact of Ramadan on that timetable."