Classroom have been created for the teachers, in order for them to be able to track their students' progress (the main purpose isn't towards learners but teachers). Therefore, when you join a classroom, the "owner.teacher" of it will be able to:

• see your email address
• block your access to forums and streams
• block the learning of some words (and Immersion if you have it currently)

Also, by entering a classroom, you'll not have any new content to the course. The "teacher" will be able to assign you goals, but no new content.

So before entering a classroom, be aware of all that and decide if it's worth for you as a learner to enter a classroom (in particular, is it worth to share your email address with someone you don't know?).