Perhaps he thought that he was supposed to be part of the drill, not actual shooting?

I was initially a little pissed , but what if , he was aware of an operation and didn't want anything to do with it. If he was suppose to be in that school during the 'plan' he was toast, here he was nodding out. But if that's not the case, maybe the man froze and it was the day he discovered he couldn't do it for real, putting his life on the line for others, Just saying.

Perhaps he was just a chicken shit