Not drinking enough water would cause headaches but wouldn't cause your vision to go. Anyway, you're far more likely to get a problem with this in the Summer when it's hot (ish).
Incidentally, most people don't drink enough water. 8 glasses a day is the recommended amount (and tea, coffee and beer/alcohol don't count as they dehydrate you). If you've been cycling you'll be sweating more of course, so it's a good idea to have some water when you get to school.