Catholic schools are the world's largest non government run school system
Are they more popular is some countries?
Catholicism arrived in Australia is 1755 and remained popular there.
Are all the kids who go there Catholic?
Most are, but I am sure there are some cases where they are not. Irish immigrants are the main source of the expanding church.
Are most of the teachers nuns or regular people?
Nuns otherwise known as sisters are usually the teachers.