Yeah, those are my thoughts regarding gay teachers - a teacher's sex life is completely irrelevant to their teaching. I can't see how it would come up in a classroom setting. With transgender, though, it can be hard to hide, especially if the person had the change later in life, so I can imagine kids might realise, especially older kids. My only thoughts are that a transgender person teaching secondary school would be pretty brave, because teenagers can be hostile towards difference, due to their own insecurities. Although if you got a 'cool' teacher who was able to relate to the kids and get them on his/her side, then that'd be brilliant for challenging kids' stereotypes and helping them be more accepting of difference.
I had a transgender professor at one uni I studied at. When I started my degree, the teacher was a male, and when I finished, she was female. Students tend to be more mature by the time they reach uni though - I just remember there being a lot of curiosity to see what the teacher would look like now she was female!
On another note, I have a friend who won't let her children meet their transgendered aunt, because she thinks they are too young to understand such things. I don't get the logic at all. Little kids have far more fluid gender concepts than adults!