The Misfits by James Howe, and the two sequels Addie on the Inside and Totally Joe.
Younger end of your age range, for kids who are struggling socially. American but I don't think it matters, in fact maybe makes them a bit 'cooler', they're about 4 friends who don't fit in. My socially awkward dd really likes them, I kind of got them for her with the 'bibliotherapy' thing in mind .
Feeling like a misfit (older pupils) - Jane Eyre / Mansfield Park / The Bell Jar And (younger pupils) Michael Grant's 'Gone' or The Curious Incident of the Dog In The Nightime or that Alex whatsit one that's big at the mo
Some great ideas so far. You're definitely right about the librarian, SnowWoman, but I'll have to wait until tomorrow and my mind is fizzing right now! She isn't doing anything as openly 'therapeutic' as this but I know will be full of ideas.
Louise Lawrence's "Patchwork People" (I think her "Children of the Dust" is on some school reading lists already, but that one too) "Daz 4 Zoe" "Stone Cold" "Brother in the Land" by Robert Swindells. I don't often recommend Jacqueline Wilson but "Sleepovers" is a nice one about a girl who is scared her friends will be put off her by her severely disabled sister.