A 17-year-old girl is in a serious but stable condition and has been transferred by helicopter to Adelaide for further treatment, after being stabbed at Renmark High School in South Australia's Riverland region.

Key points: Student stabbed at Riverland school

Student stabbed at Riverland school Police and paramedics called just before 2:40pm

Police and paramedics called just before 2:40pm An 18-year-old student has been arrested

Police and paramedics were called to the school — which was placed into lockdown — just before 2:40pm.

Police arrested an 18-year-old female student, who has since been charged.

The woman from Paringa, also in the Riverland, is accused of aggravated assault causing serious harm.

Police said that she would also be flown to Adelaide for the treatment of an unspecified medical condition and is expected to face a bedside court hearing on Wednesday.

The helicopter took off from Berri shortly after 6pm. ( ABC News: Haidarr Jones )

The SA Ambulance Service said the injured student was initially transported to Riverland General Hospital in Berri.

She was then flown to Adelaide early on Tuesday night.

The ABC understands the students involved were in Year 12.

Students moved to gym during incident

One student told ABC Riverland that teachers kept other students in the gym for their last lesson until it was safe to leave.

"Thinking about it, someone got stabbed, it was scary, but the teachers kept us calm," she said.

"They had to keep us calm, safe in the gym. I think I felt safe in the gym."

Parents said the school had not informed them of the incident.

In a statement the South Australian Education Department said it was working to ensure support for students and staff and it did not believe there was a "specific threat to the wider school community".

"The Department for Education is liaising with the school to ensure appropriate responses are put in place, including counselling and support for students and staff," the statement read.

"An assessment is being made as to whether additional security measures are required.

"The lockdown has been lifted and the general student population released to go home."

Police and paramedics were called to the school at 2:40pm. ( Tom Coull: ABC Local )

Renmark High School principal Mat Evans published a letter for parents on the school's Facebook page confirming the lockdown.

"We share your shock at this event which we are treating with the utmost seriousness," he said.

"While the police investigation is taking place I am limited in the information I can provide however I will attempt to answer any questions you may have."

He said the school would provide an update to parents and caregivers on Wednesday.

The ABC reached out to the school for additional comment.