The Girl You Know by Elle Gonzalez Rose
When Luna's twin sister, Solina, dies suddenly, Luna knows it wasn't an accident like the police are trying to make it out to be. Luna suspects Solina's death has everything to do with Kingswood Academy, the prestigious and reputable boarding school that Solina attended-- the school she wanted to drop out of with only one semester left. Kingswood Academy doesn't know that Solina is dead, so Luna takes the opportunity to pretend to be Solina and investigate her death.