The teenager was at a new school for the new school year. But as it turned out, he was not ready: His bangs were too long.

Having failed a much-feared rite of the Thai public school system — the hair inspection — he was subjected to another: His teacher grabbed a pair of scissors and clumsily chopped his hair.

“My classmates were all staring at me, I felt so embarrassed,” the 15-year-old from southern Thailand said. The incident in May, he added, remains “a scar in my heart.” He asked not to be identified for fear of reprisal from his teachers.

For decades, public schools in Thailand have policed not only students’ manners but also their looks. In addition to uniforms, students are required to either wear crew cuts or trim their hair at their ears. Dyed hair is not allowed. Running afoul of these rules can mean an unwanted haircut in class, a humiliating experience many Thais vividly remember into adulthood.

Students have long campaigned for loosening the rules, which were introduced by a military government in 1972. It was a key demand when they took to the streets in 2020 to protest the military-backed government in power then.

Finally, in March, a top court overturned the hairstyle directive, which had been enforced by the Ministry of Education. Now schools set their own policies. But students say these rules remain oppressive, especially in rural areas.

In July, at a school in Ratchaburi Province, west of Bangkok, teachers cut the hair of 50 girls. The offenders were wearing their hair past the name tags on their uniforms. Once the incident became public, the school issued an apology and vowed to review its disciplinary rules.

To many, the issue is about nothing less than democracy.

“Those in power want to turn us into citizens who are easy to rule,” said Laponpat Wangpaisit, who started a group called Bad Student, where children can report what they consider rights violations suffered at the hands of teachers.

Thailand is a constitutional monarchy that holds regular elections. But the will of voters is often rejected by an unelected establishment comprising the military, the judiciary and the royal family. Critics of the hair policy say it is a way to make citizens compliant from an early age.

“To them, happy citizens are not worth more than people who obey,” said Mr. Laponpat, 22.

The Ministry of Education declined to comment.

Despite the court decision, Mr. Laponpat’s group receives at least one complaint a day, most of them about hair, he said. He shares some of them online, but most students choose to remain anonymous, fearing a backlash from teachers.

Watcharin Keawtankham, a hairdresser in the western town of Mae Sot, said that he regularly fixes botched haircuts administered at school. Chopping the hair of students, he said, is a way to humiliate them.

Last year, Mr. Watcharin posted a photo on Facebook that showed a big bald spot under the otherwise full head of hair of a young boy. It was punishment from a teacher.

“I felt sorry for the kid,” the barber said. “The only way to fix it was to give him a really short buzz cut.”

The Supreme Administrative Court said that hair regulations were harming the mental health of children. Acknowledging Thailand’s openness to L.G.B.T.Q. people, it explicitly mentioned those with diverse gender identities.

But that did not help Auto, a 15-year-old transgender girl, who asked to be identified only by her nickname.

She used to wear a short wig to school to hide her long hair. On the monthly hair inspection day in May, teachers were measuring students’ hair with a ruler. One of them discovered her wig and threatened to burn it. Intimidated by her teachers, she went to a salon that evening for a haircut.

“I felt lost,” Auto said, sitting on the floor of her parents’ living room on the outskirts of Bangkok.

There are some schools, like Wat That Thong High School in Bangkok’s upscale Ekkamai neighborhood, that are more liberal about students’ hairdos.

“It is their head, nobody should dictate what they want to do with it,” said Kaokorn Suksangiamkul, a teacher who was sporting a buzz cut after recently ditching his man bun.

The mullet, he said, has become a fairly popular hairstyle among students.

Phatit Kalaphakdee, 16, goes by Rin and wears a middle part, curtain bangs and a pony tail. She transferred to Wat That Thong High this year because her previous school threatened to punish her if she didn’t cut her hair.

“Hair can give you confidence, or it can ruin your day,” said Rin, who is transgender.

On a recent Friday morning, students were crowding at the main gate of her school, where a teacher was inspecting their fingernails. Those with long or dirty nails were sent back to the end of the line and told to use clippers hanging nearby on a string.

Rin was among those told to cut their nails. She shrugged it off and complied. It is her hair, not her nails, that she wants to keep long.