Same as before, I don't want people wearing specific stuff just to optimize farming!
I was thinking you were against stat boosting clothing specifically for combat - to prevent something like a 'every player equips one of a very small selection of equipment in the end game like a Sword of the god slayer and armor of invincibility' situation - but thought something that reduces time spent grinding (without cheating or setting the characters attributes higher then normal at the beginning) out of combat might be a good fit.
One of the characters even mentions wearing a school uniform makes it feel easier to study (or something close to that) so I did not think it would be excessive to suggest it.