Three young people were killed and two others critically injured when their car slammed into a tree Saturday night in southeastern New Brunswick. (CBC)

Flags flew at half-mast outside the high school in Riverview, N.B., on Monday, as students and staff mourned three teens who were killed on the weekend after their car slammed into a tree.

The five people involved in Saturday's crash in southeastern New Brunswick were all students or recent graduates of Riverview Senior High School.

Counsellors and psychologists were at the school Monday to help students deal with the tragedy.

"What we try and do is help people that need to grieve, to grieve," School District 2 representative Allen Marr said Monday.

Five passengers were in the vehicle when it crashed on Route 905 in Forest Glen. An 18-year-old woman, a 19-year-old man and an 18-year-old man were pronounced dead at the scene on Route 905 in Forest Glen. An 18-year-old man and a 17-year-old girl were taken to hospital in critical condition.

Riverview principal Douglas Prescott described the students involved in the accident as being full of life and dreaming of bigger things ahead.

"I know two of the families personally, as well, so it was a great shock to me, and certainly impacted me in the sense of disbelief at first," Prescott said Monday.

"Your second thought goes immediately to what is my role as principal and how do I begin to support those families who both, unfortunately, have lost a loved one, and those families who have children, students, in the hospital."

Full investigation

Caledonia RCMP Sgt. Marco Papillon said it appears that some of the teens were not wearing seatbelts.

"Four people were ejected from the vehicle," Papillon said.

Police are conducting a full investigation into the crash, including the possibility speed may have been a factor, he said.

The RCMP have not released the names of those killed and injured.

As the school and community grieves, they are also remembering the teens' lives.

They were "students who impacted our school in many different ways, from being involved in school activities to varsity sports. Students who were excited about future opportunities," Prescott said.

"And for those in the hospital, we certainly wish them the best and hope that they recover completely. Our thoughts and prayers are with all the families involved, especially with those who have lost loved ones."

Counsellors and psychologists will be at the school all week.