OAM, it won't let me quote that, so I'll answer the three things.
Sigh. Are you limited to being so literal? A "high school schedule" simply means it's a weak schedule.
We're in this college football world where everything has to be sort of extreme. Your schedule can't be mostly good. It has to be high school teams or UTTERLY BRUTAL. A coach has a very narrow window between trash and great. Everything is bombastic. I try to keep a more even keel. Thus, I roll my eyes at the idea that a weak schedule should be characterized as high school. It's also an outgrowth of the fact most people have the most trouble understanding what Division I is. If that wasn't a thing, I'd probably be easier on it.
We know there are 65ish P5 schools, so one of them having a SOS in the 70s is, in any context you want, embarrassing for them.
Reeeeally poor example - the weak SOS allowed them to have a good record and get into the playoffs.
So these kind of go together. You throw around the 70s as if you know what it means. And you don't. And in truth, you don't want to dig, and have context. You want a number that sounds extreme. We know this because you think the Eagles' schedule was the reason they made the playoffs. Which is silly and inaccurate. But you're not super interested in that part. You want to find the number and have it mean what you want it to mean. (Interestingly college football reference doesn't seem to have season-by-season lists of SOS, so we can't go into how numbers can bunch in the middle)
So here's a hint of background. That SOS is a bastardized NFL metric. It pairs with a simple rating system. The point of the SOS is to say how many points per game your schedule is above or below average and then cross that with MOV. So last year, the Eagles with the bad SOS were the best team in the conference. The Pats were the best team in the other conference. The gaps the spots in SOS are variable and probably pretty thin at points. In short, it's not the magic number that says what you think it says.