The Tk'emlups te Secwepemc First Nation said the remains were found with the help of a ground-penetrating radar during a survey of the school.
The indigenous children, some as young as three, were students at the school, which was once the largest in Canada's residential school system.
"Many other First Nations who had residential schools within their communities also want to learn and want to use new technology to be able to find their loved ones", she said.
Survivors started calling her Thursday when the discovery was made public, saying they can't sleep because the reports triggered awful childhood memories, she said.
The central government took over administration of the school in 1969, operating it as a residence for local students until 1978, when it was closed.
Trudeau wrote in a tweet that the news 'breaks my heart - it is a painful reminder of that dark and shameful chapter of our country's history'.
The Kamloops Indian Residential School operated from 1890 until 1978 and was the largest school in the Indian Affairs residential school system.More news: Ontario moving up second COVID-19 shots
Casimir adds leadership of the Tk'emlups community "acknowledges their responsibility to caretake for these lost children".
Di tribe say dem don contact di home communities wey na dia pikin attend di school.
The results are preliminary with a final report expected at the end of June, but more remains could be discovered, she added.
British Columbia's chief coroner Lisa Lapointe told Canadian broadcaster CBC "we are early in the process of gathering information".
The discovery of a mass burial site on the grounds of the former Kamloops Indian Residential School could lead to a fresh wave of litigation, but don't expect anyone to go to jail. The government was committed to "memorialising those lost innocent souls", she said.
In a statement, British Columbia Assembly of First Nations Regional Chief Terry Teegee called finding such grave sites "urgent work" that "refreshes the grief and loss for all First Nations in British Columbia".
Canada's minister of indigenous relations, Carolyn Bennett, said residential schools were part of a "shameful" colonial policy.More news: New Zealand extends pause on travel bubble with Australia's Victoria state_china
They are working with the community and partners such as the BC First Nations Health Authority, to provide resources and the support needed as determined by the community, the ministers said.
"That this situation exists is sadly not a surprise and illustrates the damaging and lasting impacts that the residential school system continues to have on First Nations people, their families and communities, " its CEO Richard Jock wrote in a statement.
From about 1863 to 1998, more than 150,000 indigenous children na im dem snatch comot dia families and put inside dis schools dem. They were forced to convert to Christianity and not allowed to speak their native languages.
The Truth and Reconciliation Commission concluded in 2015 that residential schools were a program of "cultural genocide".
How and when the children died is still unknown.
"These were children - all belonging to a family, and community, and a Nation - who were forcibly stolen from their homes under the authority of the Canadian government, and never returned", Phillip said.
Hundreds of pairs of shoes were lined up along the steps of the Vancouver Art Gallery Friday as a tribute to the hundreds of children whose remains were recently found on the grounds of a former residential school.More news: Cummings opens fire on ministers over pandemic