Why? Simple really, if a student is hardly getting any energy from foods and drinks, then they sure as hell aren't going to do as well in the exam as when they had a constant supply of food and water.
And in modern society, where the grades you get are very important to get into university courses, medical schools, law schools, which are also necessary to get into many higher paying jobs, then fasting for a full month, during exam time, could -blam!- a few people over unfairly.
Now this isn't an attack on Religion, just a small observation which came about today in my Biology class, so if you think I'm just trying to attack Islam then have fun trying to say that I am.
And yes I'm aware that Ramadan doesn't always fall on times of the year where exams are happening, but there are years where it happens.