In 2009-2010 school year there were 98,817 public schools in the USA. Approx. 83 million students in school in 2011.
In 2013 there were 9,393 schools in Australia with 3.6 million students.
So the USA has 10 times the schools and about 20 times the students. I'm not going to count all of the school shootings but at a glance the ratio looks about right at somewhere between 10x and 20x.