Background information (minus the complex words!)
When you get a cut and start bleeding, your blood is supposed to form a clot, essentially just acting like a natural bandaid. Your blood can't just do it by itself, there has to be "ingredients" that combine to create it. A bleeding disorder means that some of those "ingredients" that help to form your clot are missing or are not doing their job, so bleeding lasts longer than it should. It's usually inherited, it isn't contagious, and it doesn't go away.
The most famous bleeding disorder is hemophilia, but the most common one is von Willebrand disease (VWD), which is estimated to affect up to 1 in 100 people, most of whom don't know they have it.
Symptoms that are across all types: easy bruising, frequent or long nosebleeds, heavy periods, bleeding for a long time after dental work or surgery, and, in severe cases, bleeding into joints and muscles.
Most of the time, when a girl has a heavy menstrual period, it's the symptom most likely to be shrugged off, by doctors and by everybody else. However, it is possible that the girl might have VWD, since heavy menstrual bleeding is the most common symptom of VWD in women. So, it might not actually be something that should be shrugged off by society. Maybe the girl might have an actual disease.
An invisible illness is a serious health condition that you can't see from the outside, such as bleeding disorders, lupus, Crohn's and colitis, POTS, diabetes, epilepsy, chronic pain, sickle cell disease, and many more. Kids who have invisible illnesses often are told they don't "look sick", miss school for appointments, and have to sometimes decide who to tell about their illness. School gets a lot easier when the adults and friends around these individuals understand. You can find more information at the For Schools page.
These are the big organizations who make a big difference!
Reminder: The information here is for awareness, not diagnosis, as I'm a student. Please bring questions about your own health to a real medical professional or a hemophilia treatment center (HTC).