Exactly what Melissa is saying. If you don't seek help, you will eventually end up hospitalized and then people are going to know. It sounds like you are ashamed of this possibility. This isn't a joke or something small. Mild or not, Bipolar gets worse and worse as it is left untreated.

Furthermore, financially speaking, you are better off getting help while you are on your parents health insurance than trying to get help, when you have no choice, but have no insurance or money.

Your friends don't have to know. But if you are having emotional highs and lows, chances are, they already notice and are simply not putting the puzzle together. Do you not have anyone at all in your life who would be supportive of you no matter what? Maybe a grandparent or an aunt or a cousin.

My last suggestion is to maybe fake a cold or something that would lead you to have a general doctor appointment. When you get in with the doctor, discuss your feelings with them and your suspicions. Express your fears and concerns about your family and friends finding out. I don't know where you live but in the US, HIPPA prevents doctors from telling your health problems to anyone.