No, never heard of cameras making a meaningful difference in mitigating / stopping school shootings. Obvious, recordings can have evidentiary value though typically there's no question who the shooter is / was.
However there is this promotional video where video sort of stops a shooting:
Classroom locks have potential to make a difference, though.
I think that even if cameras were utilized effectively, I'm not sure you'd hear about it because that's not really "news".
In any scenario like that, more situational awareness is never a bad thing, but if I was a school spending money on reducing the probability of fatal activities I wouldn't spend the money on interior cameras first.
It is awfully tough to prove a negative. I did see video once in my life where a bank robber came into a branch with video, left, went across the street and robbed a branch that did not have it.
We install cameras in branches for retail banking. We always recommend a camera on the night deposit. Problems and complaints about lost deposits, robberies etc are reduced dramatically.
Given the mental state of a school shooter, I would doubt he/they would care. I do find it interesting that most, if not all are male. It is very rare to hear of a female school shooter, regardless of age, race, etc.