Yeah they have omitted the subject because they already included it, which makes the passage quite confusing.
But since they started with saying "the first school opened...", you can assume the final comment is referring to the number of schools since that seems to be the main point of the passage.
Because the second sentence is missing a subject next to the word "ten", you have to go back to the first sentence to find the subject they are talking about, because it can't be city, the other subject.