I wanted that old school feeling of power combined with clunky old wonky cards (eg ramping out a craw wurm with black lotus) but also wanted the decks to be focused and have reasonable curves. Powered, but 1 on the Serra Angel scale. To that end, I started with only cards on the old school list and added sets chronologically until I reached the desired power level. Everything up to and including Odyssey block has that old school flavor for me, so I would have been fine going that far, but I reached the sweet spot already at Visions. I added Weatherlight just to finish the block (and give it that extra dollop of color fixing).

Apologies to the creators of the actual format "Middle School", which has a larger card pool than I used; I came up with the name before I was informed of the existence of said format, and will change my cube name as soon as I come up with another name that's almost as good.