A year eight student at Adelaide's Unley High School is one of eight new people to test positive for COVID-19 in South Australia today.

Key points: A letter from the school principal was sent to all parents and caregivers

A letter from the school principal was sent to all parents and caregivers The principal said the school would remain open despite the second case

The principal said the school would remain open despite the second case The school had been closed after a staff member tested positive earlier this week

SA Health is still investigating how four of the people contracted the virus.

In a letter to parents, the school said the student had been identified as a close contact of a staff member who tested positive for the virus last week.

The school reopened its doors this morning, after closing for cleaning once the staff member's infection was confirmed.

But the letter states the school will not be closing again in light of the student's positive test, because the child was not at school during the infectious period.

"It is important that you know that SA Health have confirmed there is no further risk at Unley High School due to this child," principal Greg Rolton wrote.

"All other close contacts of the staff member are currently in quarantine and are being monitored for symptoms."

The total number of positive coronavirus cases in South Australia is now 50.

As well as the year eight boy, SA Health said three of the new cases were a man in his 20s who travelled to Europe, a woman in her 50s who was in the United States, and a woman in her 40s who was a close contact of another case.

Chief public health officer Nicola Spurrier said four other cases were "under investigation".

"That doesn't mean we are saying it's a community spread — I'm just simply saying we are seeking more information about their contacts and travel history," Dr Spurrier said.

The State Government has launched a COVID–19 Command Centre at the SA Health headquarters in Adelaide's Hindmarsh Square.

SA Health, SA Police, the SA Ambulance Service, fire services, the State Emergency Service, Primary Industries and Regions SA and the Department for Environment and Water will all have representatives in the office.

Inside the COVID–19 Command Centre at SA Health's headquarters in Adelaide. ( ABC News: Isabel Dayman )

Student stayed home while sick after contact

The ABC understands the Unley High School student attended school-based events and visited a number of businesses while suffering from symptoms last weekend.

After confirmation of the staff member's positive test, the school was initially due to reopen on Thursday, however, the closure was extended for a further 24 hours.

The school had reopened to staff and students on Friday morning. ( ABC News: Lincoln Rothall )

The family of the student has spoken exclusively to the ABC, and confirmed the student "had contact with the teacher in a classroom at school early last week".

"They did attend a school camp later in the week with other students and staff, but the [infected] teacher wasn't present at that camp," the family said.

"They felt sick over the weekend and were kept home from school on Monday, tested on Tuesday, and had the positive result come through on Thursday.

"They didn't have any symptoms at the camp, and no knowledge of the confirmed case until Monday."

Mr Rolton said while the second positive case was "not ideal", the school was confident the isolation process had been effective.

"We will continue to monitor the situation via SA Health. I will keep our school community informed," he wrote.

A letter distributed to parents by SA Education Minister John Gardner on Wednesday stated that the protocol was to close a site straight away if a case was confirmed.

"The Government's current COVID-19 protocol is to close a site immediately for a minimum of 24 hours if a confirmed case is identified for someone at that site," he said.

"If that happens parents and carers will be contacted to come and collect their child and/or agree how they can leave safely."

A number of parents are collecting their children from Unley High School early today after the announcement.

Parent Jodie Murphy, who collected her daughter this afternoon, said the school should close.

"I probably think they should," she said.

"I would have picked her up if I had heard about it today just until they figure out what they're doing."

Dr Spurrier defended waiting until this morning rather than last night to inform Unley High School families about the new case.

She said she understood why parents might be upset but she wanted to make sure the email that went out was accurate.

"If anyone held that up, it was me, and the reason for doing that is I wanted to double-check the process," she said.

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SA Health supportive of schools staying open

Unley High is among several Adelaide schools to close because of coronavirus cases.

A student from Sacred Heart College and a Scotch College student have also tested positive.

Both schools were temporarily closed to allow for cleaning to occur.

Earlier this week, acting chief medical officer Michael Cusack said SA Health supported schools staying open and "advocating for parents to send their children to school".

"The information we have from the largest review of COVID-19, which has taken place in China, was that although children undoubtedly will get the infection, they did not find evidence of that infection occurring inside the schools," Dr Cusack said on Tuesday.

"It wasn't the case that children were infecting each other at school. It was much more the case they were getting the infection in their own families."

An Unley High School senior keeping his distance from other students. ( ABC News: Lincoln Rothall )

SA Health said there were concerns that, if schools closed, students would be left in the care of grandparents and older relatives.

"If children have the disease, we're exposing perhaps the more vulnerable members of our population to COVID-19," he said.

The Government also revealed two hospitals were set to be reopened in Adelaide, to boost bed capacity by almost 200 to deal with an anticipated spike in cases.