I ask this question a lot here in Thailand... mostly to my students.
Yesterday, during Flag Time, which is when we sing the Thai national anthem and pray together and do announcements and all, the Thai homeroom teacher for my class did something different.
She walked around and asked each student to hold out their hands. With some of them, she would just keep going. With some, she would tap on their knuckles with a thick marker. I could hear it, and it didn't sound very good. Sounded like it hurt.
I asked one of my students quietly what she was doing, and he said he didn't know. One student started crying because the marker hurt her.
The Thai teacher continued on, through the whole class. I figured out somewhere in the middle that I think she was inspecting the length of their fingernails.
When we got back to class, this was confirmed after I said, "Can anyone tell me what Kru Noi was doing during flag time?"
Someone said she was making sure their nails weren't too long, according to the Student Handbook.
Interesting.... well, now I know.