Bonjour Tim,

"Je suis en classe" means that (generally speaking) I am in a lesson somewhere in school/college. It would be the same in the expression "Sarah est en classe".

"Je suis dans la classe" means (specifically) that I am in a specific classroom rather than in an indefined classroom in a school/college. It would be the same for "Sarah est dans la classe". 

I hope this is helpful.

Bonne journée