The crash involved a bus carrying 36 children and a silver Skoda Octavia car driven by a 45-year-old woman, with two child passengers, aged nine and seven.
It happened at 4.45pm on Saturday and resulted in four people being taken to hospital with serious injuries.
Inspector Greg Dinnie of the Road Policing Unit said: “Our enquiries into the cause of the collision are continuing and I would urge anyone who saw what happened, and has not yet spoken to officers, to come forward.”
Emergency services attended the scene and the driver of the car was taken to Aberdeen Royal Infirmary with serious injuries.
The two young passengers of the car and a 48-year-old man, the co-driver driver of the bus, were taken to Raigmore Hospital, also with serious injuries.
It is understood the car burst into flames following the collision.
The rest of the people involved in the crash – the 64-year-old driver of the bus, 36 secondary school children and their teacher – were not seriously hurt and did not require medical treatment.
The northbound and southbound carriageway were closed for some time for investigations surrounding the crash to be carried out.
Inspector Dinnie added: “I would also ask motorists who were using the A9 at around 4.45pm and may have been recording via dash-cam devices to check their systems and provide any relevant footage to us as soon as possible.
"The road was closed for approximately 10 hours to allow our crash investigations to be undertaken. I would like to thank the public for their patience whilst this was being carried out.”Anyone with information is asked to contact police on 101 quoting incident number 2981 of June 19, 2021.