residential schools

mass graves were never mentioned

Despite the use of the term "mass graves" by media outlets around the world, First Nation chiefs never mentioned the term.
the facts

they are unmarked graves

The term "mass grave" has deep and significant meaning to many around the world, particularly those who survived the Holocaust. To many, the term brings up images of genocide and execution, but the term has become more loosely defined over time by some organizations, like the United Nations. 
In this case, it is important to note that the chief of the Tkemlúps te Secwépemc First Nation, Rosanne Casimir, specifically stated in her formal press conference that "this is not a mass grave". 
Chief of the Cowessess First Nation, Cadmus Delorme, said in a CTV interview, "This is not a mass grave site, this is just unmarked graves."

news outlets called them mass graves

Despite First Nation chiefs never using the term "mass grave" and specifically stating that they are not mass graves, including the chief of the First Nation where the original grave site was discovered in Kamloops, several news outlets across Canada called them mass graves in their coverage. 
Many outlets that used the term in their original reports have since retracted the term from their stories, like the Toronto Star, which included an editor's note stating, "Editor’s note: This story was first published May 31. This version removes the term 'mass grave,' as that description was inaccurate."
Many outlets have yet to modify their usage of the term. 

timeline

May 27th, 2021 - Tkemlúps te Secwépemc First Nation announces finding unmarked graves using ground penetrating radar.
May 29th, 2021 - The New York Times publishes an article entitled "Horrible History: Mass Grave of Indigenous Children Reported in Canada"
May 29th, 2021 - The Associated Press runs a syndicated story in several news outlets around the world entitled "Mass grave with remains of 215 children found at Indigenous school in Canada, an 'unthinkable loss'"
June 7th, 2021 - The Hill Times publishes an article entitled "Kamloops Indian Residential School Mass Grave Is A Crime Scene"
June 11th, 2021 - The city of Victoria, B.C. cancels Canada Day celebrations
June 24th, 2021 - The Cowessess First Nation announces the discovery of 715 more graves
July 12th, 2021 - The Penelakut Tribe announces the discovery of 160 unmarked graves
July 15th, 2021 - Tkemlúps te Secwépemc presents a detailed report called "Kamloops Indian Residential School Le Estcwéý (The Missing) Report" on its ground penetrating radar findings
July 15th, 2021 - Tkemlúps te Secwépemc announces that it is considering an excavation of the grave sites

(timeline to be progressively updated)