The three most seriously injured students were taken by air ambulance to Edmonton, said Kerry Williamson, spokesman for Alberta Health Services, which runs St. Therese Healthcare Centre in St. Paul, where the remaining five were taken.
Three of those five injured students have since been released from hospital, he said.
Two of the seriously injured students underwent surgery and remain in critical condition. The third student taken to Edmonton is in stable condition. The driver of the minivan — a 46-year-old man from St. Paul — was taken into custody.
RCMP said the man received minor injuries in the crash and is in hospital.
"There's various aspects of this investigation that we are looking at and ruling in or out," said Chief Supt. Randy McGinnis.
"One would be impairment, impairment by alcohol or a drug, and the other reason for this accident could be a mental condition."
Witnesses told CBC News that before the crash, a white minivan was racing through back alleys in St. Paul. McGinnis said there was only one person in the van at the time of the crash.
RCMP said the vehicle left the road and drove into the side of the classroom where Grade 6 students of Racette Junior High School were taking French. The class was being held in the old Glen Avon School building.
"I heard a big boom," said Grade 7 student Sydney Cross. "The lockers collapsed and smoke came everywhere ... and I heard a lot of screaming. I'm still kind of scared because there was screaming. Glass flew out of the door."
Three children were pinned beneath the vehicle, police said.
"It was really traumatizing," said student Kyra Fontaine. "I was worried about who was in the classroom because you know so many people when you're going to school in such a small community."
Fontaine said students were "bawling" when they learned about how seriously one student was injured.
"You never think it would happen to your school and when it happens you get pretty shocked."
People stressed and scared
"My children said they heard a loud crash," said parent Michelle de Moissac. "My son specifically said he heard a lot of screaming, which was the scary part.
"And then teachers just frantically — they have certain teachers there that are there for emergency like that, have the first aid and such — and so they were all running to go see what was going on," she said.
"But he's very upset. He was freaking right out when I picked him up."
The minivan entered the classroom and dropped about two metres, disappearing from view outside the school, said Robin Tremblay, a woman who works near the school.
"I just see broken windows. It's wide open, it's definitely wide open. People running in and out, like firefighters and police officers."
Tremblay's friend was in the classroom above the crash scene at the time, she said.
"She's really stressed and scared. She's been texting her mom telling her she's OK. She said it sounded like an earthquake."
"Seen two stretchers come out," she said. "Pretty sure a larger stretcher and two smaller stretchers.
"I'm hoping everybody's OK. I'm praying nobody's seriously injured."
School closed on Friday
The school was evacuated after the crash and students were bused to three other schools in the area, said St. Paul education superintendent Glen Brodziak.
The school will be closed on Friday to allow officials to evaluate the structural integrity of the building.
If necessary, officials plan to move all 250 students to the regional high school in St. Paul.
"It's important at this time to keep them together," Brodziak said.
Brodziak commended the actions of staff for how they responded to the crisis.
Emotion crept into his voice during the Thursday afternoon news conference as he described his reaction as a parent.
"There's probably a lot of parents in this room and it is hard to imagine. When we send our kids to school, there's a level of expectation for all of us," he said.
"That troubles me. That troubles us."