Hi,
From what you describe this could be the beginning of your secondary breast development. It is also called thelarche.
It is usually associated with a firm tender lump directly under the nipple, and can be unilateral to begin with.
It signifies the onset of puberty in girls. You need not be worried about it.
You should talk to some adult or your family doctor about this in detail to clear out any doubts that you have.
Let us know if you have any other doubts and post us about how you are doing.
Hope this helps.
Good luck.
If you're young enough to be in school, you must be in your teens, right? If that's true, then your lump is very likely to be NOT cancerous. The first thing that comes to any woman's mind when she finds a lump is "cancer," but there are all kinds of lumps that aren't cancerous too, especially for young women. You should still ask your school nurse to check it out, though. You won't make a fool of yourself at all. The nurse will probably be very happy that you're developing the good habit of checking your breasts while you're still so young. Please go and talk to her. I think you'll feel a lot better afterwards.