I don't disagree personally, but it's a dealbreaker especially in a state like MA. I think that would make the perfect become the enemy of the good: the more onerous you make this, the more likely it is that schools would never do it. The training expense would be a good excuse for them to half-step, as would any other kind of expense.
More requirements are a worse answer than fewer requirements. If you must, just take interested teachers and turn them into auxiliary cops; problem solved.
A key is deterrence through publicity: allowing teacher carry automatically makes schools harder targets, just because shooters will be less likely to strike at a place where it's known return fire is a probability. They'll find a softer one. And then we'll be hearing about hospital shootings or something.
Just repeal the law. That's all.