I too cycled to school in the 70s and don't remember anyone using a rucksack. It was either a briefcase, a satchel or a sportsbag (usually a big Adidas vinyl affair with handles but sometimes smaller affairs with shoulder straps). The number of cyclists at school must have been in the 100s; briefcases and satchels were usually on a spring clamp rear carrier (occasionally satchels were carried over the shoulder or even worn rucksack stylee with a bit of clever manipulation of the strap). If the large sports bags weren't carried on the rear carrier, they were usually rested on the top tube (always referred to as a "cross bar") and the handlebar stem/handlebars, with the knees going up and down either side of it. Some people had front carriers. The girls' school bikes almost all had baskets on the front.As others have said, each to their own.
I used to commute to senior school with old style rucksack, in the 1970's, subsequently commuted to work, so that's over 45 years commuting, always with a rucksack. Quality over the later years has been considerably improved on the old school rucksack of four and half decades ago, and I've always, luckily, had shower facilities at my places of work. As such never been a problem for me and never contemplated panniers.