Well, 'tu pyar hai kisi aur ka' implies that it's a love that is for someone else. In a school love story context, it could be about a person who has feelings for someone else in the school. Maybe it's a story of unrequited love, where the protagonist loves a classmate who doesn't feel the same way. It could also be a story full of jealousy and longing as the person watches their beloved be with others.
The main plot could be that of a heartbroken student. They discover that the person they like is in love with someone else. So they have to deal with the pain of seeing their crush with another person every day at school. They might struggle to move on and find themselves constantly comparing themselves to the other person their crush loves.
I'm not sure specifically which 'main khayal hoon kisi aur ka' you are referring to. It could be a song, a poem, or something else. Without more context, it's difficult to tell the full story.
If it is a song, without listening to it or having more details about its lyrics and the context in which it was created, it's hard to give the full story. But generally, a song with such a title might be about unrequited love or a sense of self - effacement where the singer feels like they are just a thought in someone else's mind, maybe longing for that person to notice them.
Since there's very little information in just this title, it could be a story related to a job in Delhi where 'didi' and'mai' might be characters. Maybe they had some sort of adventure or encounter while working there. But it's really hard to say for sure without further details.
I'm not entirely sure as the name seems rather unique. It could potentially be a very personal or region - specific novel. Maybe it's about a character who is seen as essential to someone's every flaw, but without more context it's hard to say for certain.