An Oklahoma company has developed a kind of protective blanket for students to drape over themselves (actually, strap onto their backs) in places where underground tornado shelters are not available at the school site.
It uses a lightweight, high density type of plastic to act like ballistic armor against flying debris as well as to help cushion blunt force trauma.
This is one article about it. No photos of the product in this article. Not cheap at $1,000 per blanket, but still cheaper than constructing a proper tornado shelter big enough for the entire student body.
I hate to say it, but considering the crazy world we live in nowadays, its ballistic qualities could also probably help in an active shooter situation in a school to buy some time. The article mentions that the blanket performs to Class IIIA level although not sure that was an official DOJ test they mention.