For my GCSE's, I remember fasting on some exam days and not on others. Now that i'm in 6th Form and have a lot more knowledge surrounding the purpose and benefits of fasting, I'll definitely fast during my exams if they clash but I would suggest you always come into school whilst fasting and if you can't pull through (maybe because of the crazy hot whether, health, etc.) then you could always break it and make up for it after Ramadan. But that should be only if you have to as God doesn't bear you with more than you can cope with but he is also the most merciful. Take care of yourself and remember the people who don't usually have a choice in this matter x