I don’t understand this joke: “They want an S if you’re single and M if you’re married…it’s a whole government thing.” Can someone explain?

Maybe says something along the lines of, for her math test name, “They want an S if you’re single and an M if you’re married, but since we’re in high school, we’re all single but it’s a whole government thing.”

I really do not understand why this is included in the script. What is its purpose? Thanks!