I don't teach middle school but I teach at a combined elementary-middle school and get to encounter a lot of Early Teen Culture, which is, as you would expect, really embarrassing sometimes. It's also really endearing to see, because it wasn't all that long ago that I myself was one of those embarrassing teenagers.
Also I guess it doesn't quite matter for light novel translations and whatnot, but Chuunibyou is not JUST delusions of magic and powers; it's the delusions that come when middle schoolers (eighth graders in Japan but maybe other grades elsewhere) are so self-absorbed and hormonal that they start doing really stupid stuff. Like pretending to have a British accent when you move to a new school, or acting like a hardcore gangster and convincing everyone that you really did see that drive-by shooting last night, or wearing a t-shirt to school every day that tells people you're in love and asks them to guess who you're in love with (I did this last one once and I will never forget it because it haunts my very soul).
So for me, use the tag anytime you have embarrassing teenage histrionics appear.