I'm sorry about your graduation night, Thornton. How sad!
I met someone on Saturday night who is in a similar situation. She is a M2F TG. Her wife has known about the CDing since they've been married, years ago. Even though the TG does not hide her clothing nor does she sneak around to go out dressed, her wife refuses to see her dressed, will not hear of it, will not discuss it, will not acknowledge that it exists.
It's as if these people believe that TGs have a choice in the matter, or in the case of your parents, they believe it is a phase. Many family members truly are in denial. In my TG friend's case, the wife won't even read material on the subject. Your parents, however, might open their ears and eyes to becoming educated, since you are their child and I do believe there is no stronger love than what a parent feels for their child.
Your parents may be just thinking it is a 'phase' and they are using the same tactics that might have worked when you were 'misbehaving' as a small child, not realizing this is for real and you ARE an adult when it comes to deciding your gender.
They need education. Are there books you could give them that explains about being TS? Or a good online internet site for young adults who want their parents' support? Is there a counselor who might intervene on your behalf? There are increasingly more parents who allow their young children to express the gender they feel. Could you research news articles or other resources you might share with your parents?
But most importantly, do not give up on them. You can continue to reinforce (lovingly) that you are not going through a phase, while reassuring them that you love them very much and you would like their support. It might take awhile, but please do not burn your bridges. For a parent, having an 18 to 21 year old child is on the cusp of realizing they are full fledged adults. As I mentioned in an earlier post, it takes a while to let go.