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How hard is it really to train a willing and motivated student to use a basic mill, to do basic operations?

It took me about 15 mins to get people to start doing bolt patterns on our in shop mini mill which has an DRO. And because all they had to do was turn a dial to .5 increments they were dead on precise. I used the CNC Bridgeport for the longer rails (anything over 11 inches) due to the extra travel and speed.

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Quoted for truth. If you can't do operations like this on the mill, why have a mill?

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Fairly difficult when the mentors and experienced students who would otherwise be doing the teaching were still learning themselves.
The mill was a fairly recent acquisition that the team had not yet had time to learn how to use efficiently, so we made do with the experience we did have.

I agree that using the mill to drill the holes would have been preferable, both in the interest of accuracy and in the interest of teaching skills. As always, it was a matter of utilizing the resources and experience available.

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If VEX can make beautiful 7075 gears for a few bucks apiece, I'd bet they could do some box tubing with holes in it for a great price too. They essentially already do this, with slightly different products.

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I would normally agree, but have you looked at the price per foot for VEX metal? They obviously haven't moved manufacturing overseas for these items but I am not sure they can get down to the $1 per foot range even if they do that.

It's currently $40 for 6 feet of 1x1x1 C channel. (This is thicker sheet metal and not extrusion but I would think it's comparable.)