Has the feature been removed?
You or your teacher should turn this feature on in the settings of the class you are in.
Weird. Sure looks like it, I can't follow you although I still have "friends" on my profile page and in my leader board on my home page.
EDIT: I can unfollow / re-follow folks that I'm already following.
MORE EDIT: I just can't follow you. Maybe because you're in a classroom ("Duolingo for Schools")?
You have "Stream" disabled. That's probably what's causing you to not be able to follow / be followed.