I guess I don't follow you. I think they had about 1/2 to 1 hour warning, most likely 1/2 hour, as the tornado wasn't on the ground all that long west of them... But no matter how much warning they had, what were they supposed to do? Go west into the storm? No. Go south or north, and then west, essentially going around the storm. No, most likely not, as the path is uncertain (how much north or south do I go, to make sure that I miss the storm, before I head back west?). Go east and try to out run the storm? Perhaps, but again, where was it heading, to make sure that you stay out of its way?
And since I think we are talking about the school kids, how on earth are they supposed to get cars and drive? And don't say the school should have: they are not about to take kids out of school and bus them around trying to dodge a tornado, without parental permission. How long would it take to get all the buses up, load everyone up, make sure that all were accounted for, and then head out? Much longer than the warning they had. Even if it worked, lawyers would have a field day with this one. Especially if they didn't outrun it...
No, realistically, taking shelter inside the building the best they could was the lowest risk action they could take.
I would have left personally after a half a mile wide tornado already struck outside of Oklahoma City and more were on tap in that same area for the next day.