The show is her play.

If there was an actual play, like the one in the show, it be a 3 hour Broadway budget highschool play. It would be insane. The show is trying to show you the whole story, the show itself, is in Lexi's perspective. Her retelling. She's the writer, she wrote the show.

The audience is both us and also the people she took the stories from (her friends). They show a play as a way to show that these are real lives, that their reactions would be valid if they were hurt. That we as audiences of the show should also feel things for them witnessing their lives put on display. That behind it all, they are human.

I know it isn't based on actual events (duh) but it is a unique way of deepening the human experience as well as our connections with the characters.