I don't know a thing about statistics, or how to calculate this- but to just hazard a WAG, the odds that one of three people in the world who stopped an attack on a French train, would be attacked themselves a few days after one of the other three people in the worlds school (where he was supposed to be that day) was attacked must be vanishingly small. I hope the third guy is watching his six. As well as the other two.
I mean, just the chances of Scarlotos school being the subject of a mass shooting is infinitesimal.
Plus-
My belief is the "authorities" would not admit it even if they knew for sure it was a jihadi revenge. There have been many instances of SJS that have been covered up.