Girl Defective is a great title, but it doesn't suit the eponymous protagonist Skylark so much as her brother Gully (short for Seagull). Gully is the active mystery solver, and the kid who never takes off his pig snout mask.
It's eventually a fun, cute story, but it took me well over 100 pages to get interested. Normally I don't give a book that long, but I read it sort of with Susannah Bartlow & Milo Miller, so I wanted to finish it. I think if you like music more than I do or are an Australiaphile, you'll get the grip more quickly than I did.
I wish the thread of Skylark's crush on Nancy had been formally recognized or resolved, but I was glad to see female friendship valued, along with connecting with the sad, but otherwise perfect male love interest.