Does this imply there's gay in their school?
Sure, if you didn't pay attention to all the flaming homosexuals.
It pretty much confirms it
I want to believe everyone is straight (except Mugi)...
None of them are gay.
the letter's from yui...
It is actually the other way around. Everybody is gay but Mugi (who is just a yuri fangirl but still straight)
So Yui's gay.