Turkish schools reopened Monday for the first time since July's attempted coup, following a summer which saw tens of thousands of teachers sacked or suspended over alleged links to the plotters or to Kurdish rebels.

As more than 18 million children went back to school, Huseyin Ozev, president of the Istanbul teachers' union, told AFP there were fears the academic year would begin with "chaos" because of staff shortages.

After a rogue military faction tried to oust President Recep Tayyip Erdogan, Ankara launched a massive crackdown, dismissing and detaining tens of thousands within the judiciary, the police and the education system over alleged links to the putschists.

Deputy prime minister Nurettin Canikli said Monday a total of 27,715 had been dismissed from the profession, while 9,465 teachers were suspended and under investigation.

But Canikli said 455 had been allowed to return to teaching after investigations showed they had no links to "terrorist" organisations.

Students arriving at school on Monday were handed pamphlets from the education ministry commemorating "the triumph of democracy on July 15 and in memory of the martyrs".

Pupils were being shown two videos about the coup, the ministry said, including footage of Erdogan reading the national anthem alongside images from the night of the coup showing tanks and war planes firing in the capital Ankara.

In schoolyards, students observed a minute of silence for the victims and a prayer was said.

- 'We are teachers, not terrorists' -

Authorities have blamed the coup attempt on Erdogan's arch-enemy, US-based preacher Fethullah Gulen, and followers of his moderate Islamist "Hizmet" (Service) movement.

Gulen, who lives in self-imposed exile in the United States, strongly denies any involvement in the coup, and the crackdown has sparked alarm among Turkey's Western allies.

Prime Minister Binali Yildirim warned teachers not to "tolerate" those working for Gulen -- whose movement it refers to as the Fethullah Terrorist Organisation (FETO) -- or the outlawed Kurdistan Workers' Party (PKK).

"Dear teachers, never tolerate among your colleagues those who serve FETO or the separatists," Yildirim said in a televised speech during a visit to a school in the eastern province of Erzincan.

Union chief Ozev warned that children's education could suffer if inexperienced teachers were called in.

Speaking to AFP, he said the school year was likely to be characterised by "general chaos" because "there are 40,000 to 50,000 vacancies and no preparation on the side of the ministry of education".

A teacher in Istanbul who identified herself only as Cigdem criticised the sackings and suspensions as she defended her colleagues.

"We won't them let do that, we will not let down our schools. We are not coup-mongers or terrorists, we are teachers."

- '20,000 new teachers by October' -

Some 11,500 teachers suspected of links to the outlawed PKK -- which has waged an insurgency since 1984 -- were suspended earlier this month.

There have been near-daily attacks by the PKK since a fragile ceasefire collapsed in July 2015, while Turkish authorities have responded by stepping up their military campaign in the restive southeast.

In Kurdish-majority Diyarbakir, nearly 100 teenage students held a sit-in to protest against the suspensions in the city's main square shouting slogans including: "We want our teacher back."

One student wore a T-shirt reading "Don't touch my teacher", and was among more than two dozen detained by police, an AFP correspondent said.

The detentions came after the students refused to stop their demonstration, which is banned under the three-month state of emergency imposed a few days after the failed putsch.

The deputy prime minister told reporters that those dismissed for alleged terror links would be replaced next month.

"By October 10, 20,000 new teachers will be able to start," he said.