Nutmeg and Kikki - please speak to a school counsellor. Any senior figure in your life is also a good starting point - maybe an elder at church? Or go to your GP - they are often able to help. Just bear in mind - if it really is bipolar (And while you should respect his professional opinion, let him know of your concern) - you need to be on a mood stabiliser before an anti-depressant. Some GPs will assist with mild forms of psychiatric conditions, but they usually end up referring you to a pdoc.
I would just imagine it would be a lot easier to convince your mom to take you to a GP (Even if you need to come up with an excuse) and can then in private talk to the GP.

I hope things work out for you. It must be so frustrating to need help, and not be able to do anything because you are waiting for your parents to assist.

But as I already mentioned - bear in mind that the teen years are NEVER easy, but things will get better