I've wondered this myself, since raising a unit's Skills to A (the maximum needed for exams) takes time. Oh, sure, you can set a unit's earned Skill EXP to be one short of a raise using the editor, and then use the Max Action Points code to instruct each and every student, but the editor only maximises the Skill EXP for the current letter grade.
For example, say I maximise Dimitri's Skills when he's got a bunch of Ds. Sure, he'll be one EXP away from D+ for each, but if I want him to have straight-As, then (since you can only save at the end of each week, not during Instruct time) I'd have to button-mash my way through D+, C, C+, B, and B+. Either that, or settle with the D+ grades for each Skill, get to the end of the week, save, use the editor to maximise Dimitri's Skill EXP again, get to Instruct again just to give him the 1 EXP per Skill needed to reach C, and repeat.
Repeat this with every single unit, save Byleth, and it quickly becomes extremely tedious, regardless of the method chosen.
A way to instantly raise (or lower) a unit's Skill grade would've been quite appreciated, had I not just button-mashed my way through.
