You get the pink apron by doing the side quest of the kindergartener in the park by helping those 2 kids with their problems.
That's the gakuran scene--the teacher-student one. You have to buy the gakuran costume from the love hotel bear and use the costume. I could be wrong, but I don't think Tocchan's sexual inclinations matter here. (If it does, Tocchan was a dom top in my playthrough.) If you don't get the CG the first time he wears it, it will probably trigger the second time he does. ^^
I finally saw the pink apron make an appearance, but it wasn't a CG or even a full scene. Tocchan's apron was getting old, so Haato offered his as a replacement. Not long after, foreplay started and Tocchan was wearing the apron but that's it. No other comments on it or anything, just that he's wearing it. Bit disappointing that there wasn't a scene for it but oh well.
Meanwhile I'm just realizing that I've had the Gakuran/teacher-student scene the whole time and could have told you that was the one you were missing (and how to get it) if I'd been paying attention. oops