ANDY'S ANSWER: 11 is still young to be getting your period. I'm not saying it can't happen, but it's more likely when you're 12 or 13.
Even when your first period does happen, it won't be like you suddenly start gushing out blood. It will just be little brown or red spot that you will notice on your underwear that won't be enough at that time to soak through to your clothes. That's why I suggest that girls who have started puberty carry a pantiliner in their purse or in their locker, so as soon as they see some signs of the period they can put it on.
For sure later your periods will get heavier, but they will always start with just a little blood that will give you warning that it's coming so you can put something on to protect your clothes.
Believe me, no one will know but you when it happens.