You think they would be more exact considering the max legal length is spelled out as 5.5".
The LEOs I know in Texas look at the knife a person is carrying usually only because the person being examined has already done something illegal or stupid to get the LEOs attention. The same LEOs all use the same "ruler" for checking knife length - a $1 bill. Any denomination US currency is 6" long. The LEOs say as long as the knife isn't longer than a $1 bill, they don't care - unless the person is going to the pokey for other charges any way.
As far as carrying knives at schools--- I don't any more. I simply choose NOT to attend any school functions - plays, sports, assemblies, etc. Among many things, I'm a beekeeper and I was asked to come give a presentation on bees. I said "Sure, as long as you can provide me with a letter of permission signed by the school system Superintendent and elementary school Principal allowing me to bring any knife I choose onto the school grounds for the duration of my visit." Still waiting on that permission slip.
Now back when dirt was clean, I carried a pocket knife to school starting the 2nd week of first grade. That first weekend after school started, my grandfather gave me my first knife - a small, single blade peanut, saying -
"Now that you've started school, you need your own knife. We can't have you borrowing everyone else's knife."
Since September 1965 (5th grade), I have carried, at minimum, a pair of pocket knives, one in each FP. This included 10+ years of active duty in the Navy (but not ashore overseas), 10+ years in the Reserves (except when sent overseas) and through 2 college degrees. I would add a pair of fixed blades when able.
This thread
http://www.bladeforums.com/forums/showthread.php/1086041-Your-Important-KA-BAR-My-Papa-s-KA-BAR
contains the back story for my carrying at least 2 knives.
Since being asked to not let the door hit me in the butt on the way out when corporate America decided to send my job to China back in 2001, I have carried, at the least, paired fixed blades and paired folders. The only exceptions to this has been when I have had to fly for business 4 times or at a court house or police station.
Knives at school?? Heck, most of the guys and some of the girls had rifles and/or shotguns in the rear windows of their pick-up trucks out in the parking lot.

But not me.

I had to mount my gun rack to the underside of my trunk lid because I had a Ford Galaxy 500 instead of a pick-up truck.

When someone got a new rifle or shotgun, the whole school new about it within a day or so - and the teachers would come out to your vehicle t lunch - to check it out.

