Je suis en classe. -- I'm in class. This means that you are attending class not necessarily that you are physically in the classroom.

Je suis dans la classe. -- I'm in the classroom. But not necessarily attending class. You could be there just to clean the board. Note, that most likely you'd say je suis dans la salle de classe.

Hi Bozena,

As the lesson explains, there is a nuance between the physical enclosed space of the classroom and the student group.

So we have "Je suis en classe"   --> I am together with my classmates as a group (perhaps outside).

"Je suis dans la classe"   --> I am physically within the four walls of the classroom space. 

There is more here to aid your understanding:-

https://www.wordreference.com/fren/classe

Hope this helps.

Jim

They are 2 distinguishably different statements in French. You are looking at them from an English speaker’s perspective - not from a French speaker’s one.