If only I could hide my face like an Ostrich!

I was substitute teaching my little brother Aaron’s class last week and a little boy came up and asked if he could use the restroom. I said, “You can go when she gets back.” and the ‘she’ said, “You mean ‘He’.” I was confused and said, “She asked first, and then you can go.” looking back to the little boy. Again the ‘she’ said, “You mean ‘He’.”
I was as confused as you probably are. The whole class was staring at me, and my little brother, sitting two feet away, started laughing. I realized just then that the ‘she’ was a ‘he’ and ‘he’ was correcting me. I was so embarrassed. I tried to cover it up and just dug myself deeper. I finally just said, “You can go after HE gets back.” That seemed to pacify the class, but Aaron was still snickering at my uncomfortableness. I glared my baby brother down, but couldn’t help but laugh myself.
Oops.
The funny thing is, that his name is Tristan and he was wearing boy clothes. Earlier in the day I made mental notes, like: “Hmm… I thought Tristan was a boy’s name” and “She sure dresses like a boy” Those clues should have tipped me off that it was a boy, but for some reason they didn’t and I made a fool of myself.