I think it depends on where you live and school budgets. I don't think it's necessarily a new thing. I never went to a school that had a nurse (back in the 80s). There was a spot in the office with a bathroom and a cot where you could go if you were sick and depending on what your symptoms were they would call your parents or send you back to class. This was usually run by parent volunteers or aides.
When we lived in NY my kids' schools had a nurse on staff. We are in North Carolina now and the school has one nurse for the entire district (county). She rotates around the schools, basically just checking their records and things like that not actually taking care of sick kids.
I really don't understand the jump to lawsuit. The kid was in class for the remainder of the day and not complaining. Why would you bring a lawsuit against the school when they really had no way of knowing that the kid had a broken arm? Several parents have posted that their own child broke a bone, went about their evening, and didn't realize until the following day that they needed medical attention. Should they all be reported to CPS and have their children taken away for negligence?
I think you're assuming it was a visible/noticeable injury and that they purposely didn't take action even though the child was in distress. But it is possible to have a broken bone and still act completely normal. My husband has broken his hand and then went and played golf. I broke my foot in three places. I knew it was injured, but didn't think it was broken so I didn't go to the doctor.