Rebecca,
PM me whenever you need to. I work in theater and sometimes the hours are weird and long but I will get back.
I find the truth of the world our kids are growing up in is typically more accepting than when we were that age. I don't know how progressive your school district is but there are support groups in many high schools. My oldest is a leader in the Gay/Strait Alliance(she has many friends on the spectrum both sexual and gender ID wise). They are open to trans kids as well. Perhaps there is a group at the school...? If so, it is easier for some teens to talk to their peers than their parents. The only problem I have run across is the teens think they have all the information but occasionally it is wrong or skewed. Is your youngest male or female? Compromise is good, but this does not go away. Eventually they will have to experience/work through this too.
I am one to talk. While my oldest knows, I have held off talking to the younger two(13 &15). I will tell them but am waiting a little(I suspect both will be wholly supportive- they are very much a trio and may be angry I didn't tell them when the oldest was informed). However, as parents, we have decided to hold off on them for another year. I told the oldest so when she comes back from college on breaks next year, it won't be a surprise.
I wish you luck, love and acceptance.
With warmth,
Joanna