It sounds to me as though you have tried in many reasonable ways to communicate your feelings to them already, but you certainly should not give up because if you are suffering from depression, you must get some help.  I wonder if the reason your family is shrugging this off is because it is too painful an idea for them to process or they don't have a great deal of knowledge about depression.  Sometimes parents feel that it might be their fault if their child has depression, but, of course, that is not true at all.  It's a biochemical process in the brain, not the fault of anyone. 

I have a few suggestions to share with you.  First, you might want to write a letter to your mom and step-dad, maybe being able to explain more clearly in writing how you are feeling and that you need some help.  Second, you might want to do a little research for them and print out some materials on how important it is to treat depression.  Third, you could ask them what kind of insurance coverage is available for you, and if there is no coverage or poor coverage for treatment, you can investigate clinics in your area, which often offer treatment on what is called a "sliding scale," meaning a family that does not have a high income pays far less.

Now, if your efforts to communicate with your family are still not successful, or even if they are,  I would really like to see you talk to a counselor or a school nurse at your school.  If there is no counselor or nurse available, a teacher you trust would be a good  choice.  Counselors are trained to help students with problems, and perhaps a counselor could even sit down with your family and explain the importance of treatment.

No matter what steps you take, please don't give up on getting yourself some help.  Depression is serious, but it can be treated, and you have a whole life ahead of you that will be a wonderful life, with just a little help.  I think we would all be happy to hear from you again, to know that you have followed up, gotten some help, and are feeling better.