There are too many variables for a 'one size fits all' plan.
The advice I drill into my kids' heads, and my wife's, is first, to do SOMETHING. There is no time for denial. If something's not right, if you see someone with a gun, you hear gun shots, screaming, we are conditioned (in our polite society) to downplay; to assume that it's nothing, it's a prank, or a hoax. Kind of like that guy who just reversed course and headed directly at me in a huge parking lot, he probably has a question or something. If I start yelling or run from him, I'll look like a weirdo. The last thing we want is to look like a paranoid psycho by reacting defensively. That hesitation can be fatal. REACT! The consequences of overreaction in a case like this are minimal. The consequences of hesitation can be grave.
Second, is to consciously break out of societal norms in an emergency. This also does not only apply to mass shootings. If you need to escape, the cost of replacing a broken window cannot be a consideration. The emergency exit you know not to use because it's alarmed. Yeah, it's still there. Can you punch out a ceiling tile, jump on a desk and escape through it? Do it. To use a non school shooting example of this in action, I hear people say all the time that they were "blocked in" on the road by a raging motorist. Your car will go over the curb. Some tire tracks across a median or someone's yard are better than sitting in your car, trying to call 911 while you're beaten. Most of us follow the rules and conduct ourselves in a way that when an emergency occurs, we may not consider reactions that would save our life, because in other circumstances they would constitute vandalism or scare other people.
The recognition that we live in a dangerous world, and the willingness to avoid threats may make it difficult to befriend random strangers one would encounter out in totally uncontrolled environments, if that ever even happens, but it will keep you safe, and minimize your chances of having to face down threats with a weapon, let alone be slaughtered. This is as true in a school shooting as it is for adults like most of us here.