He isn't a teacher, not really
It almost seemed funny, to Blaine, the idea of love at first sight. The belief that you could look at someone and from across the room you'd know instantly that it was love. They are the one. And you don't even know their name yet.
Although the idea stopped being funny to Blaine and slowly became realistic one glee club rehearsal.
He'd was texting Cooper when it happened so he didn't notice when he entered the room, he hadn't even noticed when stopped talking and went to greet him. It wasn't until he said "guys this is an ex student who has agreed to help you prepare for Nationals" that Blaine looked up intrigued.
As soon as Blaine looked up their eyes met and everything seemed to freeze for that second and even when the man had looked away he couldn't stop staring.
Kurt. His name is Kurt they are later told.
Blaine knew that it was probably wrong to feel like this around Kurt but he isn't a teacher, not really.
Kurt came back twice a week to help them prepare for Nationals and Blaine didn't know if he'd loved Kurt the first moment he saw him, or it was the second or third or fourth. But three weeks in Blaine was certain, he was in love with Kurt.
On that third week Kurt and Blaine were alone after school in the choir room rehearsing Blaine's solo and he couldn't contain it anymore.
Their lips met in a sweet kiss for a moment before Kurt pulled away to protest. Blaine shook his head and reassured him that they weren't doing anything wrong before connecting their lips again.
It was okay because Kurt isn't a teacher, not really.