i see a lot of scared parents in the comments, and I can understand being worried, middle school is always a scary time, but I am begging you to not potentially make it worse for your child. If they are scared to tell you their trans, ask yourself if there is a reason, if you have given them a reason to think you may not support them. 
if you are arguing of science, I beg you to do research (I beg all of you that this term scares to do some real scientific research) as it is more than just people who experience ‘being trans’. It happens in other species too, but for them they are not burdened with high brain function and a community that may mock and hurt. 
Trans people are not new either, being open about it is, and that is wonderful, as it means trans people may get help and be proud of who they are. 
For those arguing with bible versus, I’d like to one, mention separation of church and state, and two remind you that the golden rule has always been love thy neighbor. How can you say you adhere to that when you can’t even love your child for who they are? God said he created us in his imagine, and that means every aspect, including the changes we make to feel more at peace within ourselves.

I have trans friends, some who’s parents are the reason they are happy and feel supportive and have always been confident in themselves, and I have trans friends who do not speak to their families for what they did and said to them when they came out.