I disagree with Raquel. While the sentence is not clear, I believe it means the city now has 10 schools which will open next month.

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.

Or ten schools

The ten refer to people.. students, teachers.. Ten hundren students maybe..

I agree with Sudsey as 10 refers to the number of schools.