The Education Ministry has instructed schools not to give students "ugly" haircuts as a punishment for turning up with long hair.
Prasert Boonruang, permanent secretary at the Education Ministry, said he had sent an urgent letter to ministry offices responsible for schools to clarify the regulation over pupils' haircuts.
In it, he told the offices to notify schools of the cancellation of the old haircut requirements first enforced in 1975.
Schools must now adhere to a new regulation, which calls for agreement to be reached among a school committee comprising the principal and representatives of teachers, parents, students and communities.
In short, haircuts must now be based on what is appropriate, said Mr Prasert.
He admitted some schools were still enforcing the old regulation and advised them to meet parents, students and teachers to agree upon clear guidelines.
Mr Prasert said it was unacceptable that a teacher in Si Sa Ket had cut one girl's hair to her ears as punishment for turning up with long hair.
The girl's mother, Chatuphon Kham-ieam, says she has withdrawn her daughter from the school after both were threatened by alumni, other students and teachers for posting news of the incident on social media.