That sounds useless [0] considering you could just cut out a hole in a textbook or hide a gun anywhere else. A lot of schools have metal detectors these days as well as random bag checks. Seems like that's the future we're choosing.
Or, you know, the shooter doesn't decide to spend the day at school before they start shooting. They just walk in the front door with guns blazing.
Clear backpacks are more for spotting drugs I think, and even then it's pretty useless. Dealers in schools aren't carrying around kilos of coke, they're carrying easily concealable dime bags and the like.
These mass shootings are mostly in rich white suburbs.. The shooters are largely white males. Good luck getting metal detectors in these communities. Even if you turn schools into a prison you will have large groups of kids lined up to get to class in morning, at sporting events, activities and the other functions. So a motivated shooter will get to people anyway. So even though high school can feel like it is a literal prison, actually making it so solves very little. The only reasonable and logical action to take is gun control. You can't ask teachers to make threat assessments, determine mental state or act be law enforcement. Teachers are there to teach.
[0]: https://en.wikipedia.org/wiki/Security_theater