They Met at Blackwell
It’s pretty obvious at this point, but it’s still an important point to bring up. Chloe and Rachel are students at Blackwell, though it is suggested in both Life Is Strange and Before the Storm’s promo materials that the two didn’t necessarily hang out in school. It’s not surprising, considering the fact that Rachel was highly regarded as an outstanding student at the academy, while Chloe was looked down upon as a rebel.
From all the released footage of Before the Storm we’ve seen so far, it’s not exactly clear when exactly they met and properly became friends. We get treated to a scene in an underground club, where Rachel bumps into Chloe, and the two appear to already have an established friendship, so it’s possible they had interacted with each other even before that.