The whole "Adolescence Syndrome" thing is essentially a narrative metaphor for maturing. Over different arcs of 2-3 episodes each in the series (Mai's being the first), different characters experience different supernatural phenomena that the protagonist helps them to process and resolve. The rest of my explanation I am going to put behind spoiler tags for people that haven't seen the opening arc of Bunny Girl Senpai. Also note, that this is just my take on things as an anime-only based on recollection of the show back when it first aired.
Bunny Girl Arc
The opening arc focuses on Mai, the child celebrity that is always in the spotlight. As she gets older and moves to a different school, she finds that it is tough to break into social groups after they have already formed. She feels like an outcast socially as well as professionally after she made the decision to leave her line of work and break off contact with her manager/mother. This feeling of being an outcast and alone manifests her version of Adolescence Syndrome in that others, in addition to simply not interacting with her socially, literally cannot see or recognize her existence. The condition spirals as Mai seemingly comes to terms with it and accepts that she isn't worth interacting with anymore. This is what Sakuta refers to as going with the flow of the atmosphere. It isn't until Sakuta forcibly breaks the flow of everybody around him by demanding their attention during an exam and causing Mai to reassess her own self-worth and giving others around her a reason to talk about her that her syndrome is healed.
As the show goes, different characters' syndromes manifest in different ways based on the particular problems they are facing at that point in their lives. It could be suffering an unrequited love, jealousy of others, dealing with bullying, or coming to terms with your changing appearance. Each of these manifests in a different way.
I watched Bunny Girl shortly after the Monogatari series and I didn't really enjoy it. I just observed too many parallels between the two and how much more enjoyable Monogatari was. Maybe my timing was just really bad.
I finished it anyway but admittedly I only stayed for the ED. :/