(a) It still felt “early” to me at the time, but not “difficult” to be up for.
(b) I don’t remember anyone complaining about school starting early or people being generally sleep deprived - if someone was exhausted in school we were all amused and curious what adventures they’d been having to stay out way too late.
(c) I do recall that there was an optional “early period” that started at 7:30 and some people took classes then or had sports stuff at that time, and I specifically remembering thinking that you had to be out of your mind to participate in anything requiring you to be up that early.
By contrast my wife, who is much more the “go to bed on time” type than I ever was, had band at 6am and school at 7:30, and describes her high school as a blur that she slept through half of.
So the article’s recommendation for a 9am start time seems pretty logical to me.