Agree, the 60's were a whole different time. For one thing, there wasn't a ton of litigation going on when someone was injured.
And the point was made in the movie that there were several hundred boys at the institution and most of them weren't like our guysaltar boys who got caught up in a prank gone horribly wrong. So the infirmary probably handled a lot of patients who had suffered a beating, both from guards and from other inmates.
And then there was the excuse of the football game.
Also, I thought it was interesting when the priest said he'd been in the same place. It sounds almost like a common thing for boys from Hell's Kitchen to spend a little time in the reformatory. I think if such serious abuse as raping and murdering kids had always been part of the protocol, the priest would have been more on the alert.
You would think, though, since Rizzo had a family who presumably had a funeral service for him, that the mortician or SOMEONE would have noticed his fatal pneumonia looked more like a beating so bad "there was nothing left of him," right?

It was a different time - in the 1960s, such things were not talked about. The book explains it all in more detail. Also, the doctors and nurses were probably in fear of speaking out, because they would more than likely lose their jobs. People just looked the other way.

Maybe someone didbut the guards could have easily said they got hurt during the game (I know improbable, but possibly said). Also, they had a history of fighting with the other boysthe guards could have laid blame on that. Basically, it's the guards' word over the boys and they all knew that. Plusthey didn't even want to talk about it amongst themselves.

Yes but then we see Shakes and Michael speaking in the infirmary. Shakes is in bed, Michael is wearing a hospital gown and says 'John and Tommy, they're on the other side there'.
The OP is correct there were too many Doctors, Nurses, Teachers and nice Guards in the school for something like this to go unnoticed. Nolkes and his buddies were just prison guards at least one of which was straight out of high school looking to make some steady cash. The genuine guardians didn't owe these guys a thing. And you have to be a complete idiot to believe the administration would have boxed up a bruised and battered kid and said 'Oh yeah must have been pneumonia'.
It's why I just can't buy the whole thing as fact.
24/04/1916

The kids were placed in solitary after the beatings, not the infirmary. And Rizzo was said to have "died of pneumonia" to cover up his death.