In "The Road Not Taken," what does “that” refer to in the last line of the poem?

Ineed with another opening line. here's the idea: a girl got detention for something she didn't do, and had to stay afterschool until evening. she headed towards the bathroom, only to be lured away by distant music. she walks down the dark hallway (which lead to the auditorium) and saw a boy playing the piano, perfectly. i just need a detailed description of this. best will be awarded brainliest. you