Does Mizuho reveal his crossdressing after he graduates?
Not really. After graduating, Mizuho maintains his female disguise only when visiting the academy. However, it's never revealed if his other former classmates know about it (though I also failed to find any remarks if they finally know about it).
According to pixiv encyclopedia (Japanese),

伝説
[...]卒業後は翔陽大学に進学、[...]
卒業後もちょくちょく学院にも顔を出している。大学では男性として、学院に顔を出す際は女装。[...]
Legend
[...] he went to Shoyo University after graduating, [...]
He was also visiting the academy occasionally. He was living as a man in the university, but cross-dressing when visiting the academy. [...]
(Emphasis mine)

According to AniOta Wiki (Japanese)

卒業後は翔陽大学という外部の学校に進学。
女装を解いてるのにも関わらず相変わらず女の子に間違われ、翔大名物美少女トリオのひとりに数えられている。
He went to Shoyo University after graduating.
Even though he stopped crossdressing, he was mistaken as a girl regardless of that and as usual, and counted as one of Shoyo's Speciality Trio Beautiful Girl.


The story related to the university life is told in the omake scenario from Yarukibako 2 (VNDB entry), a fan-disk for Caramel Box's works (the original visual novel).

 Yes, seriously, that's a guy (potentially NSFW: Mizuho in girl's underwear with his natural long-hair)