I was very tempted to respond to this last night, but decided to wait.
Mums are an immovable force. You have yours on your side now just be grateful she doesn't tell everyone what you did on the potty at two, or show the baby pictures. That's mums for ya.
At the stage you're at, it really is about you and how you see yourself. Being wrong gendered might be a little annoying or it could be a little amusing, but only if you know you're being wrong gendered.
Question a teenage boy's sexuality, (not recommended, just posing), and he will probably become quite aggressive. Same thing really. He really isn't sure yet.
Now, you need to just remember you're a nice looking guy with poor taste in sunglasses. If someone mis-genders you, then you could joke back something along the same lines. But either way, it has to be a little funny.