Millions of pupils in England and Wales are returning to school after the unprecedented shutdown during the coronavirus pandemic.
Schools will look different, with one-way systems, screens keeping pupils apart and staggered start times.
Many pupils will be given inductions so they understand the new rules, such as staying in their "bubble" groups and where to use social distancing.
Teachers will assess how much pupils need to catch up after a long absence.
Schools are beginning to return in England Wales this week - although there will be a mix of starting dates, with some schools having training days as staff prepare for new safety measures.
In Scotland and Northern Ireland schools have already opened for the new academic year.
Godsway Dzoboku, principal of Outwood Academy Portland, in Worksop, Nottinghamshire, has re-organised classroom desks into rows, with each child being given a personalised pack of rulers, pens and other classroom equipment.
"The only way we can make bubbles work is to keep them [the pupils] in year groups - so we have a bubble of 300," Mr Dzoboku said.
Each bubble will stick together, go to a staggered lunch and a staggered break together, he added, and each group has been allocated a corridor within the school in which they will stay and be taught.
The view from the first day back
By Judith Burns education reporter
There's something of a party atmosphere outside the gates of Riverside School in Barking, east London.
Nervous Year 7s say goodbye to their parents who have accompanied them to their first day at their new secondary school.
Parents are not allowed inside and the children line up in the playground in single file and in silence.
Year 11s and Year 13s greet each other and then head inside to begin two days of diagnostic exams.
"It may sound a bit cruel but we need to know where we're at," says head Andrew Roberts.
The exam years are the ones he's most concerned about, he says.
"Today we're assessing them," he says. "To get an idea of the impact on each individual."
He says they have missed three months of learning since the lockdown began - and they need to be ready for crucial GCSEs and A-levels in less than a year's time.
Delani, aged 11 says he is feeling "pretty OK" about his first day.
"I was quite scared but I've seen a lot of familiar faces," he says.
He has a face mask in his pocket but the school has decided they're not needed.
His mum, Heather, a university administrator says she's fairly confident that the benefits of being back at school outweigh the risks.
From what she has read, she says the pandemic could go on for a very long time. So the children need to be back in school and the parents need to get back to work.
Imole, aged 11, says coronavirus "messed everything up". He was sorry to miss most of his last year in primary school, even the end of year SATs tests.
He's standing at the gates of his new secondary school - a bit apprehensive. "A lot of stuff is going to change," he says.
Aisha, also 11, is "nervous and excited". "It's scary moving from a school where I knew everyone to a school where I don't," she says.
Mum Rasheedat says she's "confident" her daughter will be safe from the virus at her new school: "I'm not really worried".
Sanitisers and screens
It has been almost six months since schools were closed by the lockdown.
They soon reopened to vulnerable pupils and key worker children, but only a fraction of the national school population returned to any form of face-to-face lessons over the summer term.
The rest were required to carry on learning from home, but levels of support and interactive teaching have been very different from place to place.
It is not clear how many parents will have sent their children back, with no official figures yet, although attendance is compulsory.
Mary Bousted, joint general secretary of the National Education Union said that, thanks to the preparations by school staff, "the signs are at the moment that it's a very calm and managed and positive return".
"There will be lots of nervous children and young people this morning," she said. "I'm sure when they get into school they will be reassured by the routines and be really glad to be back with their teachers and their friends."
But teachers and head teachers have criticised the lack of funding for safety measures, such as hand-washing stations, sanitiser, and screens.
Association of School and College Leaders general secretary Geoff Barton said it was "hugely disappointing" that these changes had to be paid for out of "hard-pressed budgets".
England's schools minister Nick Gibb has said funding for safety measures is "under review", although Ms Bousted said the government was acting too slowly.
Mr Gibb has also urged parents to send their children back to catch up on missed lesssons, saying: "Schools are doing everything they can to make sure that their pupils and their staff are safe."
Some recent polls suggest families are keen to see children back in class, but others have not been so positive.
In Scotland, where pupils returned several weeks ago, official statistics show one in 10 pupils is absent, although ministers and teachers have suggested cold and flu viruses are mostly to blame.
But there are parents who are worried.
Mother of two, Sally, has a chronic form of blood cancer and has been shielding since March.
She told BBC Radio 4's Today Programme she was keen for her children to get back to school to continue their education and see their friends.
"But I do have concerns that it's exposing me to additional risks."
When they return home school, she said: "I won't be giving them a hug, unfortunately - I'll be keeping my distance as much as I can, otherwise it will be life as normal, hopefully."
Pupils returning to secondary schools will find whole sections of school buildings cordoned off to some groups, but open to others as head teachers strive to keep groups apart.
And there will be reduced movement of pupils around schools, with classes and year groups staying in a small number of rooms, while their individual subject teachers come to them to teach.
Pupils can be required to wear face coverings in busy areas, such as corridors, and lessons like physical education will be very different, with all contact sports still banned. Many will already be wearing face coverings on public transport to school.
And there are concerns about the impact on schools if there are further local outbreaks.
If a school has a positive case it will work with local health officials to assess how many other pupils should go home and isolate.
It could be just those who have been in close contact with the pupil, or an entire "bubble" group, which could be an entire year group in secondary schools.
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