How exciting! And yes, it can be so confusing. I once moved back to the region I grew up, but I left when I was 15 and returned when I was in my 30's (married and with a child). You have that false feeling of 'going back home' but you soon realise that a lot has changed in 15 years! LOL And the way you looked at things are also very different if you are a teenager, or a married mother. Hihihi.
The Fraser Institute is a good way to start. They publish the report cards for the public and catholic schools, and you can find it by going to google and search for
'Report Card on Ontario's elementary schools 2011'
That will lead you to a .pdf where you can search for 'Mississauga'. It starts on page 36 of the document.
It shows you the enrollment, how much ESL students or Special Needs students. It shows a 5 year evolution of the results on the Reading/Writing/Math standarised tests. And the average income of the parents.
Of course there will probably be 'bad' schools where your children will be happy, or good schools where they don't feel at home. But at least it is a start!