I think they pretend for the bullying so as to not get caught, Japan is very homophobic and a lot of people seem to forget that just because Yuri is a genre in their media, but irl coming out won't go down as well as it does in these mangas/animes or like in places like Canada or the USA, I can speak from personal experience when it comes to homophobia in school at least.
That said, yeah, it is kind of dumb to go to such extents to not get caught, they can simply completely ignore each other or keep it as friends for show, couple in private, but I'm guessing the author simply wanted to explore a theme like this, it's not like it doesn't happen, a friend of mine and his bf from HS pretended to completely hate each other, but in reality they were going out, they were simply afraid of getting outed, no one ever suspected a thing thanks to it until they got to college at least, if they ever got caught I'm sure they both would had been bullied relentlessly, specially bc one of them was the captain of our football team and the other was in the basketball club, teenagers can be really cruel and gay people getting beat up in the locker rooms was pretty common.
At least it is getting better and I hope Japan gets better as well, their LGBT movement is getting more attention for sure.