Basic* - the dev simply chooses if they want to make their app available (app store public) to institutions at a 50% discount by selecting the first edu option via Rights & Pricing, where blocks of redeem codes are purchased as required. You'll see those as edu purchases in your sales reports.
[ ] Available at a reduced price for educational institutions
Not sure where you're seeing an opt-in for "Apple School Manager", tho, it's not a choice when I look...
*I'm still looking for the old docs that used to cover 'basic'. Might be buried in the paid agreement.