From now on, it's military discipline all the way, Sergeant Nunes told them.

They will enter into classrooms single file -- hair short for the boys, a bun for the girls, he said.

Their school, the CED 07 educational center on the outskirts of Brasilia, is one of four public schools -- 7,000 students in all -- that are being transformed into military-run schools.

President Jair Bolsonaro, a former paratrooper, promised to establish more of these state-run military secondary schools during his election campaign last year.

By the end of the year, there should be around 40 of the new military schools, a spokesman for the Federal District's education secretariat, Mauro Oliveira, told AFP.

"We are here to empower the teachers," Captain Newton Araujo, a military police officer in impeccable blue uniform, says as he watches his subordinates instruct the pupils in the new school regime.

"We are here as collaborators, not usurpers," he said, using distinctly military jargon.

Military police have been put in charge of discipline and administrative functions, while lessons are given by civilian teachers.

- 'Military culture' -

Sergeant Nunes explains to a boy how short his hair must be cut with electric trimmers to comply with the new rules.

"Setting two for the sides, four for the back," he says.

Over the coming months, the students will make other changes. They will wear uniforms, with girls permitted only "very moderate" accessories. They will stand, each day, for the national anthem. They will receive civics classes.

"Through the principles of military culture we are going to apply rules of civility, patriotism, citizenship, ethics," said Newton.

Lucas Monteiro, a 13-year-old student, hopes the discipline will reduce "physical and verbal aggressivity between the pupils, such as beatings and bullying."

The schools were chosen for the pilot program in part because they are in poor, high-crime districts, Newton said.

They aren't the first of their kind in Brazil. There are 120 other state schools that have been operating along military lines for years, half of them in the state of Goias near the capital.

Bolsonaro, during his campaign, said he wanted that model replicated on a much larger scale, deeming them a proven success in inculcating law-abiding values in students.

Newton asserted that pupil behavior improved under the strict rules of conduct, and so did academic scores.

- Outrage and concern -

But the initiative has divided educators and outraged some.

Carla Alcantara Souza, a teacher and the daughter of a military man herself, is adamantly opposed to the model being "forced" on CED 07.

"I am extremely worried when our disciplinary director says that people who do not adapt to this model can leave. For an educator, to hear that 'if you don't fall in line, you can go,' it's painful," she said, surrounded by schoolyard noise.

She also fears that students' sense of identity would be warped by the experience.

"We are going up to a student with an enormous black afro hairstyle, accepted up to now, and telling him, 'Unfortunately from now on you have to fit into this model in which your personality is no longer accepted,'" she said.

Some students also expressed concerns.

"I'm afraid because this will affect our freedom of expression," said Maria Eduarda Lacerda, 14, explaining that she has been told she can no longer keep her hair short nor dress as she wishes.

- 'Conservative' logic -

Newton responded by insisting his military police were not taking over entirely.

"A lot of people say the military police are going to take the place of the teachers, take over decision-making powers. The truth is totally the opposite. We want to empower them," he repeated.

A University of Goias professor who has studied military schools for the past 20 years, Miriam Fabia Alves, questioned some of the ideas underpinning the change.

"Is it possible to grant authority to another? That doesn't happen. Authority isn't transferred, that is a false discourse. These schools implant an extremely conservative educational logic," she said.

She added that she believed it was part of a hidden agenda to hand over public education to the military. Bolsonaro's education minister has promised to create a special department handling just military schools.

"Today the planets are aligning in that way ... I have no doubt at all that it's tipping towards an expansion of this kind of school," Alves said.