Making mistakes is universal. It happens to everyone. If you use the wrong pronouns the best thing to do is to apologize, correct yourself and move on. Making a big deal out of it or going on about how sorry you are might just make the student uncomfortable. Here's an example of a teacher correcting themselves and moving on.
"Alright class, it's Alex's turn for show and tell. She, excuse me, they brought in a scrapbook. I apologize, Alex. Why don't you tell us about your show and tell item?"
You might also want to ask students who else they are out to, and if they would like you to correct other students and staff if you hear them using the wrong pronouns.