They don't.

It's not about teaching engineering principles (that's for A-levels and Uni) it's about teaching drawing, CAD, presenting idea's, basic introductions to common manufacturing materials the kinds of tools you'd use for them and why. A bit of learning about the basic mechanical engineering processes (cutting, shaping, forming, polishing etc) and a bit of engineering history and you're done.

Think of it like a introduction to what engineering is about, rather than learning how to be an engineer.

Which really is quite a good idea. Spark an interest in kids and they're vastly more likely to pursue it than if you just force them to do maths.

Frankly I think all 14-16 year old education should be based on sparking career interests in kids rather than testing and grading them.
Let them learn general education up to 14, then spend 2 years basically teaching them about career options.

THen let 17-18 (A-levels/technical college) worry about teaching them what they need to know to actually advance in a career.

Wish I'd had that when I was 14-16 then I'd never have wasted a decade bouncing around different jobs having no idea what I wanted to do with my life. Or maybe I would but maybe I'd have discovered a passion earlier.