This will give you an idea as to the "How to go about it".
If you are desirous of using public schools, which is the NORM, then, read through these pages appropriate to those schools.
If you are of a different denomination, and desirous of following the Catholic Schools, then, please follow the appropriate sections posted by them on their website. You will see them within the search. If they are not visible, I will post the link at a later date.
If you are affluent, and can afford special education and pay for it, then, there are private schools that you can contact for their fees and curriculums, with them, and see if they will provide any advanced standings for those children of yours. They will provide the evaluations if you send them the children's current standings at your end. Some of the links have been posted by others prior to my posting. Hence they are repeated.
Elementary Education, upto Grade 12 is free in Ontario. Most of the schools provide the corse materials at the start of the year, FREE. If the student lives two Kilometers or more there are school buses, that will pick them up from your door front or close by location, (within 500 feet) and transport them back and forth to the NEAREST available school location. This is free in the outlying areas. But in the GTA and local schools, it will be wise to ask and find those answers, after landing, or through e-mails to them. They also get bus passes at a reduced rate. Look for TTC
http://www.toronto.ca/ttc/schedules/index.htm
http://www.toronto.ca/ttc/fares.htm#faresgeneral
Look for Student.
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http://www.edu.gov.on.ca/eng/curriculum/
http://www.curriculum.org/csc/library/profiles/gr9.shtml

Specific to public schools: Grade 9 and above:
http://www.curriculum.org/csc/library/profiles/9/essentialenglish.shtml
http://www.curriculum.org/csc/library/profiles/9/essentialmath.shtml
http://www.edu.gov.on.ca/eng/document/curricul/secondary/oss/oss.pdf

Fine arts:
http://www.edu.gov.on.ca/eng/general/list/college.html
http://www.ontariocolleges.ca/pls/portal/url/page/TOPLEVELPAGES/OCAS_ProgramSearch_Search

There are many colleges such as Sheridan, near missisauga,
Georgian college in the Barrie AND Owen Sound area,
There is a George Brown in Spadina-Davenport area and the list goes on.
The locations of these institutions can be obtained if you google for them.
Please look into these prior to your arrival as you do have lots of time.
You can also see the list of these educational institutions, ranked, in Macleans Magazine, mostly the Universities, if that is the route you would like to take.
http://www.macleans.ca/universities/?gclid=COPxyfKU-4YCFRTpPgodJU8tHg
http://www.cs.hku.hk/~tse/topten.html

No education is ever lost. How they give credit for such subjects and in their placement, for a start of their education locally here in Canada, they do the evaluation of each individual student on their merit and ability. You will also do better if you meet with a carreer councillor at the appropriate locations in these schools where you would like to further your children's education here in Canada and make the decisions together. (On the spot) You will get to know, first hand, the concept and the avenues to pursue with locations, costs, and other help and assistance the schooling and education system provides for thier furtherance. Outstanding students receive admissions first, in and at any of the Higher educational institutions.
Please bring with you their ranking in the school, if available.
Any letter of recommendations. How outstanding they were in education, sports or extra curricular activities with you. If possible their curriculum attested by the school principal (Hd.Teacher) or a printed matter at that location, in case you need to do the comparison after landing here.

Good luck to you all and read through all of the documents presented by all of the previous posters here. Happy hunting.

Freddie.