Incidentally, rather than squeezing existing schools beyond their real capacity, why don’t growing communities open new schools?
Not incidental to the thread at all. You’ve just defended the existing schools very well. Even if, as jbaldy22 says, there’s a communal responsibility to educate all children (without financial consideration), that responsibility does not lay squarely on the shoulders of the existind schools.
The answer to your question, incidentally, is that it is easier to expand an existing school than to start a new one from the ground up.
New schools do open, but probably not as many as we need.