Girls, need your help. Its about mother natures monthly "gift" if you know what I mean?

I'm most defiantly a virgin and my period hasn't come for awhile now. It is never regular and it annoys me because one week I have symptoms and I'm almost sure its coming and then I get cramps and stuff and it never comes, then one day it comes and I notice it when I get home from school but luckily its always light when it appears then gets heavy. I just don't want to be in class and it starts and catches me off guard. Is there any way I can be able to calculate close to when it will come? Or am I bound to just sitting around waiting for it to arrive?
Girls, need your help. Its about mother natures monthly "gift" if you know what I mean?
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