Complex sentence: Suddenly everyone becomes aware of a gorgeous blond who has entered the room, Serena, Blair's ex-best friend (they drifted apart after Serena left for boarding school) and is rumoured to have been expelled from her boarding school.
Split into simpler sentences: Suddenly everyone becomes aware of a gorgeous blond who has entered the room, it is Serena. She is Blair's ex-best friend (they drifted apart after Serena left for boarding school) and is rumoured to have been expelled from her boarding school.