Liscar and Glebe are the two best high schools in Ottawa and they are all in downtown. It's very easy to get registered. First you have to settle down near one of these two high schools. E.g. 311 or 360 Bell St. South. Then take your child to the "family evaluation centre"- I forget the exact name-to decide which grade your child will be in; then they will ask you to take your child to have TB test (You can do it in a nearby walk-in clinic). Your child will only be allowed to go to school after you get the TB test result. That's all.
Lisgar doesn't offer ESL program, as new student from China, I don't think he/she can get into Lisgar without at least 1 year study for ESL somewhere else