I hindsight those would be right things to do.
But again: Maybe the kids went in there and the coach just followed with them. We assume that he was in charge, but we don't know. We don't know if he encouraged them to go in, or whether he tried to talk them out of it or something in between. If he encouraged them to go in, we don't know if he thought they would just enter the opening of the caves.
Some reports say that kids in that area regularly go to the caves. Maybe that's just what you do around there, like kids in America hanging out at a parking lot (which probably kills more kids annually) or kids hanging out at pools (in which way more kids probably drown than in caves). Maybe it's not considered so reckless.
Except as a tourist a couple of times, I have never been to such caves. I wouldn't think rain would be an issue. Like you, I have now read in the media that there were warning signs that said something about flooded caves during monsoon season, and maybe they saw the signs or should have seen the signs. Or maybe people in that area know that local caves flood during monsoon. I don't know. Three weeks ago I certainly wouldn't have thought of flooding as the obvious risk in a cave. I would probably have worried more about falling or slipping or snakes or earthquakes or getting lost.
And even if they did check the weather forecast, would they have known that caves would be flooded? In the last few days, every meteorologist in the world has looked into whether that mountain in Thailand would get rain and experts have tried to guess what it would mean for the flooding in the caves and the ongoing rescue mission. But although it rained it didn't mean that the water raised much. Maybe these people go to the caves regularly while it rains and it's not a problem.
We simply don't know enough to blame the coach.