“They will drink only a glass of milk before leaving home,” said Huang Xuan, the sisters’ mother. “They won’t have any appetite if they wake up too early.”

Huang said they would have a proper breakfast after they reached the school, probably around 8am.

Huang's husband is a Hong Kong-born architecture designer who is now working for a Shenzhen-based property developer. The couple decided to send their Hong Kong-born daughters to schools in the city because publicly-funded schools in Shenzhen do not admit children born in Hong Kong or Macau.

Huang said the quality of private schools in Shenzhen varied and that she preferred her children to receive a Hong Kong education.

“Sometimes I feel too sleepy and fall asleep on the bus,” said Wang-ka. “Crossing the border every day is hard. But I feel very excited to be back at school tomorrow because we can play many games, play with toys and read books.”

Ho-chun also said she had been looking forward to her first primary school day.

The sisters put what they need for school today – textbooks and summer holiday homework – into their school bags last night and went to bed around 8.30pm.

Huang said school ends at 3.30pm and Wang-ka can return home at around 4.30pm. The long journeys mean the sisters will not be able to join many afterschool activities.

“It's a loss for them,” she said. “There are some very interesting activities they like a lot. But after they get home, finish their homework and have dinner, it will be time to go to bed again. They don’t have time to do other things.”

Sometimes, she said, there were many parallel traders crossing the Futian border, clogging up the control point and stalling the border-crossing process for schoolchildren.

She said earlier this year, a new chief was appointed to the Shenzhen side of the Lo Wu border crossing and the control point had become much stricter in scrutinising parallel traders, so many of them chose Futian to move their goods across the border. She said they often chose the time when schoolchildren were crossing because the increased flow of people meant that officers at the border might not have enough time to check them.

“I just hope they won’t block the children,” said Huang. “Many of them came in taxis and there were so many goods that drivers couldn’t close their boot properly. Once it took me two hours to cross the border because there were too many of them there."

The number of cross-border pupils almost tripled from 9,899 in the 2010-11 school year to 24,990 in 2014-15. A total of 11,774 pupils crossed the border every day to attend Hong Kong primary schools in the last school year.

Officials expect the number of primary school cross-border pupils to peak in 2018-19 after the government banned mainland women from giving birth in local hospitals in 2013.

Last year, the Education Bureau implemented a scheme to spread the intake of cross-border children across a wider area amid anger from local parents over a shortage of primary school places for their children in North District and Tai Po – the districts closest to the border. Until then, schools did not set aside places specifically for pupils living on the mainland, so locals had to compete against mainland parents.

This year, three primary schools in Tsing Yi joined the scheme, meaning it now includes 127 schools in eight districts offering 2,612 places for new cross-border pupils this school year.

Some schools in areas such as Wong Tai Sin and Tuen Mun said that the scheme allowed them to open more classes at a time when schools were facing a shortage of local students due to the low birth rate.