The question is: is this "magic in the air" due to active (kids and instructors using magic) and passive (wards) magic use, or some inherent magic in the location itself? If its the former, then it would stand to reason that places like Diagon Alley would have a similar, if reduced, effect due to the number of wards around the area keeping it from the view and access of muggles, and due to shopkeeps (like Fred and George) using spells to enchant various goods. If its the latter, then what is it that makes the school special, and not some place like Diagon Alley?
The only explanation I can think of that would work in conjunction with the second hypothesis is that Hogwarts was constructed on or near a natural wellspring of raw magical energy. Channeled and focused magic wouldn't disrupt muggle electronics, unless specifically designed to do so, because the intent changes the properties of the raw magical energy, turning it into a form that is, if nothing else, less likely to cause electromagnetic interference. However, the raw arcane power, since its more chaotic and variable in nature, would have a more pronounced effect on devices relying on electromagnetism, like cell phones.