I had done the same mistake. I drafted an apologetic cover letter, alongwith revised background form, photocopies of US visa rejection letters and sent it to my Visa office in New Delhi via courier.
Generally after your Medicals, your local VO does criminality check, wherein 1 of their checks includes verifying your information with the US immigration database too (generally for asylum, deportation etc.) but your student and tourist visa rejection will definitely crop up there.
Although, that's not a base to jeopardize your Canadian PR, but they might reject your case based on misrepresentation...now...after you get a PR and land in canada..or maybe even after you acquire a Canadian citizenship...you never know..quite uncertain situation to be in....
Hence, I suggest you send your local VO all these docs by courier. Declaring this now won't affect your PR application for sure..a 100% positive on that!!
Cheers!!