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A children's doctor, who's been giving the government advice during the coronavirus pandemic, has told Newsround it is safe for pupils to be back in class.

Professor Calum Semple is a member of Sage (the Scientific Advisory Group for Emergencies) - that's the group of experts which advise the government on important coronavirus decisions, such as deciding that children can return to school.

Schools in England reopened on 8 March. Prof Semple has told Newsround how that decision was made and why he hopes it will be the last time schools are forced to close.

"I'm happy to say it's safe," he said.

"First of all, we do know and we've known right from the start that [most] children are not severely affected by Covid.

"We also know that children come to harm by not being in school because they miss out on educational opportunities and they miss out on seeing their friends.

"It's not just about the children, it's about their school teachers as well - adults do transmit (spread) the virus at workplaces and that is a real risk for them, so looking at the school altogether it's really important that we make it as safe as possible."

If you're in primary school in England, you may not be asked to wear a mask or take regular Covid-19 tests.

But if you've gone back to secondary school you might have to wear a mask and you might've noticed coronavirus testing or been asked to take a test yourself. They're sometimes called lateral flow tests and are part of plans to reduce the risk of Covid spreading in schools.

But why are the rules different in primary school compared to secondary schools?

Prof Semple explained that as children get older, their bodies become more like adults and there is slightly more risk of getting ill from coronavirus. There's also evidence that older children are more likely to spread the disease.

"Primary school children don't seem to become infected and don't seem to infect other people as much as secondary school children, so they don't really need to wear the masks, because there's really little point or benefit in doing it.

"Also [in primary schools] part of normal children's development is seeing other people's expressions.

"It's a bit different in secondary schools - it's very hard to manage a 12 year old in the first year of secondary school differently to an 18 year old in the final year of a secondary school, so the blanket decision is that secondary school pupils should wear face masks."

Outside of school in England and Wales, children under the age of 11 don't have to wear face coverings. In Scotland, anyone over he age of five has to wear a covering, and in Northern Ireland children over the age of 13 are required to wear one in places such as the supermarket or public transport.

At the moment there are tests to see if all children can or should be vaccinated.

The scientist behind the UK's Oxford-Astrazeneca vaccine told Newsround in February that a spray or pill version could be developed to help children get vaccinated.

"We'll look at using a nasal spray. We'll also look at giving the vaccine by a pill that you just swallow," Professor Sarah Gilbert said when she responded to questions from Newsround viewers concerned about kids who are scared of needles.

However, Professor Gilbert warned: "It's going to take some time, so it's not going to happen for probably a couple of years yet."

Prof Calum Semple added that he isn't sure younger children will be vaccinated at all: "At the moment, there's some research studies going on to look at look at how the vaccine affects older teenagers and some children may end up being vaccinated as part of research studies, but it might be quite some time before we see secondary school pupils being vaccinated."

Prof Semple also explained that the vaccine means schools have a good chance of staying open for good this time.

"We hope [schools can stay open], but we can never say we're absolutely sure," he said.

He also explained that most adults who can become seriously ill with coronavirus have now been vaccinated.

"So many of you will have had parents and grandparents and aunts and uncles that will have been affected by Covid, now that we have a vaccine, the people that are most likely to be affected have been vaccinated.

"Although we will keep seeing cases of Covid in our communities and in our families, it's less likely that people will be severely affected and because of that schools should be able to stay open."