I'm so sorry about this because I know that their first reaction to your condition causes more anxiety. Maybe tell a school counselor? Do you have friends who are understanding that you can talk to? Sometimes talking to others is helpful. I find the information at this link very helpful:

How To Help Yourself Get Through Depression on a Day-To-Day Basis

If you are extremely suicidal, maybe try calling someone such as:
Teenagers, call Call the National Suicide Prevention Lifeline at 1-800-273-8255 or Covenant House NineLine, 1-800-999-9999 and/or read this:
(Those are phone numbers I saw at the link below)

Suicide: Read This First

I am glad you want help. Is one parent easier (less judgemental), to confide in than the other? You might want to start with that parent. I am sorry you are going through this. I would try to tell a parent again. Maybe they were having an "off" day the first time you mentioned it or need time to accept that you are having problems with depression. I hope they come around. I am glad you are posting here. Many of us know what it is like to deal with suicidal feelings. Those feelings are a sign that you need help. I hope it gets better for you!