Thousands of children, many still traumatised from losing homes and loved ones, returned to class Sunday as Nepal's schools formally reopened following a devastating earthquake that claimed more than 8,600 lives.

In many cases children in uniform walked through rubble to attend lessons in temporary classrooms made of bamboo or in tents on playing fields, after their schools were destroyed or badly damaged in the quake that struck on April 25.

Eight-year-old Sahaj Shrestha clung to his father as they arrived together at the gates of the state-run Madan Smarak School in the Kathmandu valley.

Sahaj's mother Mina Shrestha said their son has been too terrified to leave their side, even to go to the toilet, since the quake destroyed their home and forced them to live in a tent.

"Aftershocks are still continuing. It is difficult not to be nervous about sending the children to school again," Shrestha told AFP.

"But the teachers have assured us that it is safe here, and at least his mind will be fresh if he meets his friends and studies," she said.

Classrooms made of bamboo have been built on the school's football field, while some lessons were held in buildings checked by engineers after the quake and marked "safe".

Teachers sat with the younger children as they drew or played, with some relieved to return to a degree of normality.

"We've been staying home for so long, it is nice to play here and meet my friends again," said nine-year-old Muskan Bajracharya.

In senior classes, students were encouraged to talk about the quake or share stories about what happened to their families.

"We are not holding any formal classes and have trained the teachers to help the children overcome the trauma of the quake and adjust to (being back at) school," said principal Govinda Poudel.

Yubraj Adhikari, who is leading counselling initiatives by the International Committee of the Red Cross in quake-hit communities, said teachers must be alert to any behavioural changes.

"It is normal for children to act differently after such an experience, but we have to keep an eye out for any signs of long-term trauma in a child," he said.

-'Education cannot wait'-

The 7.8-magnitude quake damaged nearly 8,000 schools, while some 90 per cent of them are estimated to be have been destroyed in the worst-hit rural districts of Gorkha, Sindhupalchowk and Nuwakot.

The quake struck on a Saturday afternoon when schools were closed. Many had been preparing to open the following week for the new semester.

"I don't even want to imagine what would have happened if it had been a school day," said Sakuntala Bhlon, 37, whose two children study in classes five and eight.

The reopening had been set for May 17, but was delayed after a second major 7.4-magnitude quake rattled the country on May 12.

The UN Children's Fund UNICEF has warned that the disaster could reverse the progress Nepal has made in education over the last 25 years, during which primary school enrolment has risen from 64 per cent to more than 95 per cent.

"The longer children stay home, the more difficult it will be for them to return to school," Tomoo Hozumi, UNICEF Nepal representative, told AFP.

"Education cannot wait. It needs to be part and parcel of the relief and rehabilitation package."

For some schools, however, reopening so soon after the quake has proved impossible, with temporary classrooms still being built and continuing concerns about safety and space.

"It is impossible for me to reopen right now. The school ground is filled with debris and we don't have an open space," said Lila Nanda Upadhyay, principal of Rupak Memorial International School in the Kathmandu valley.

Dilli Ram Rimal, education department director general, said he hoped more schools would reopen in coming weeks.

"We understand that not all schools have the resources to reopen," he said.

"But education is an important part of the recovery and we need to begin the process."