@seynee In that case it was not just a matter of staying in the closet, she pretended to like Yuu specifically so Tsubasa wouldn't attempt to pursue him in a sort of "sis code", it was just to prevent them from getting together. This probably took precedence over not being bullied by classmates, and there is also the possibility that "persecution" for going after the popular guy would be less severe than being exposed as a lesbian. Even the day of Ayano's injury she interjected solely to prevent Yuu from confessing to Tsubasa, as she just wanted to keep them separate with this pretense.
This reveal was clearly planned since the beginning as there are some details that in hindisght make more sense this way, as for example I always found suspicious that there was not an immediate reaction when Ai apparently kissed Yuu, especially as it was unwanted from his part so it was clear it never happened and Tsubasa just misunderstood, and how Ai always had more meaningful interaction with Tsubasa than him.
tl;dr yes she could have played along as friends, but she specifically wanted to avoid Tsubasa and Yuu becoming a couple
Also it works as an ulterior parallel for Tendou, who pretended to like Yuu for ulterior motives out of "desperation".