Depending on the facts in your situation, and the one
canuck_in_uk asked about is perhaps the most important one, your age, you may have H&C reasons for which IRCC will allow you to retain your PR status.
While there are some alternatives, the primary procedure to follow is to make an application for a PR Travel Document and in that application present information in support of H&C reasons why Canada should allow you to keep PR status.
For IRCC information about applying for a PR Travel Document see
http://www.cic.gc.ca/english/information/travel-document/index.asp
In the meantime, there are several other topics here in which this process is discussed, and a number in particular which are specifically about individuals who had PR status as a minor and who want to return (come) to Canada
SOON after becoming an adult. Not all such applications are granted, but it appears that the odds of getting a PR TD are very good for those who make the PR TD application within a year or so after reaching 18 years of age, perhaps even a little older.
A PR TD would allow you to board a flight coming to Canada, and upon returning to Canada, to then live and work and attend school as a Permanent Resident of Canada.
Among topics where this is discussed in some depth is one titled "Left Canada as a Child. PR Residency Obligation Violation" which, as I recall, was started by
rish888 who has started several topics and discussed this issue at length, and who has apparently done a fair amount of research about it. The following quote is from that topic, from a post by
Leon (among the more respected and reliable contributors in the forum); I have also made some very in-depth contributions to that particular conversation, but my posts there were largely focused on coming to Canada via the U.S. (using a U.S. land border crossing PoE to come to Canada) without first applying for a PR TD, which is rooted in the particular facts and circumstances of that particular situation.
In any event, the quote below should have a link which would take you directly to that conversation.
Depending on your age, making the PR TD application may be a time sensitive matter, the sooner the better. So you may not want to linger much at all. However, H&C based applications can be a bit tricky, so it would be a good idea to at least read through other discussions in the forum about this.
What not to do: Do not attempt to apply for a new PR card.
You should NOT apply for a new PR card before coming to Canada and being in Canada
You can consider when to apply for a PR card
after you are in Canada.
If somehow you travel to and are in Canada, and you can make the PRC application while you are in Canada
-- -- AFTER you have been in Canada at least two years, or
-- -- when you feel comfortable about making the H&C case in an application for a PR card
Alternatively, if you have applied for and been issued a PR TD based on H&C reasons, and using the PR TD you have come to Canada and are in Canada, you can probably feel comfortable about making the H&C case in an application for a PR card.