Didn't find anything in wikipedia, but stumbled upon the Strand archive, cited here: 1959
Yup, that which you have pictured, is not rated. Take a look at it, you will find that not even the "heavy duty" ETC clamps have an SWL listed on them. As was mentioned it has to do with the fact that they are cast iron, very brittle. This is one of the reasons that it is very bad to over-tighten C-clamps. The angle of force that the bolt applies can bend and weaken the clamp, and eventually, as someone pointed out before, they just fall apart.
To politely disagree with you...
If they need it for a special purpose, there's an 8" wrench in the tool box. But I haven't found a high school girl yet who can't sufficiently tighten a C-clamp with a 6" wrench... and I've taught a lot of them. The problem comes when a guy uses the 8" wrench to tighten it and the girl has the 6" wrench to loosen it.
The one that broke on me was brand new. It looked fine. Once broken, it was easy to see why- there was a void inside the casting. I would assume (and you know what that spells) that once it has been used awhile, the chance of that happening is very unlikely. Still, I doubt they will ever be magnafluxing C clamps, so it's just something to keep in the back of your mind. I am referring to the old cast iron ones here, not things like mega-clamps which I think have a much better QC. I just remember how surprised I was, and that has stayed with me.
I'm not talking specifically about tightening C-clamp bolts. I'm talking about, for example, the yoke bolts and the nuts on Cheeseboroughs.
To be fair, I've not spent a lot of time working with HS students (though I do have one as a full-year job shadow right now). I just seems to me making them use 6" wrenches is a short-sighted way to protect equipment. It doesn't actually teach how to USE the tools appropriately.
My $.02. And seriously, no offense intended.
Continuing the debate...
I do have to admit I cheat and use a wide jaw 6" wrench as that is needed occasionally for the yoke bolt. But my feeling with that is you should crank it down tight on the bench and not touch it in the air. Along with that, the last thing you want is someone using a 8" or larger wrench on the tiny pan lock bolt (a.k.a. "Jesus/F-Nut").