penlo_deuph, I couldn't fit my children's books recommendations into a comment, so they're behind this cut!
I used to be a bookseller for children's books, and I could write you a catalogue!
I'll try and separate them into sections, but first a point - apply to somewhere like Borders, because as a school you'll get 10% off books etc, but also many of them have authors and events for kids all the time.
Books for reluctant readers:
The Artemis Fowl Series by Eoin Colfer. I can't stress how good these books are. I'm 20 and doing an Eng lit degree now, and my bf and I have read all the series.
The Terry Pratchett Discworld Books. He's done a couple for kids now and they're marvellous. They're full of spice and argument and energy, as well as magic. Good for reluctant girls too - his girls are very rarely uninspiring characters.
Back Home and Goodbye Mr Tom by Michelle Magorian. Back home and A Little Love Song were some of the defining reading of my youth, and it never hurts for kids to read about the experiences of children their own age in the past. Surprisingly easy reading, but hugely valuable.
Bridge to Terabithia by Kat Paterson I might have got the author's name wrong on this, but the book is ideal for kids who are a bit dreamy or disconnected - it's about two children who create their own little world in the woods, and the heartbreak and pain when tragedy strikes. The colour and touch and pure realism of this book have stuck with me my whole life.
Bloodtide by Melvyn Burgess. I wouldn't recommend this for under-15s, as the content is gory, harsh, explicit, and unrelenting. It is still one of my favourite books - I am reading the newly-released follow-up, 'Bloodsong', and reminded that despite Burgess' fame with 'Junk', 'Doing It', and 'Lady, My Life as a Bitch', to me, this is the absolute height of his art, as well as the hardest read of his output.
The Deptford Mice Series by Robin Jarvis Sometimes it's hard for kids to connect with those around them. These wonderful books clearly drew the author in too, as he's written three even bigger books detailing the history of the Deptford mice. Full of the tender touches of an adult undertsanding of love, and loss, these books nontheless capture the wildness and temper of adolescence.
I'd always recommend the Discworld books for any child over about 11/12 - they're funny, rude, disconcertingly intelligent, and they give children a sense of understandinbg the world about them, sharing a private and rather intellectual joke.
Robert Westall, Dick King-Smith, JK Rowling, Anne Fine, Michelle Magorian - Most things by these authors will capture children's hearts.
For kids who love reading/feel like they're being kept behind their potential:
The Melvyn Burgess books are fantastic - not only full of amazing and readable emotion, but also a warning of what happens when you grown up too quickly.
Lian Hearn's 'Tales of the Otori' Trilogy - I was a part of a youth opera group for six years (up until this summer), and I made everyone I could read these books - they're full of Japanese culture and history, but based enough in myth and magic to enchant. I devoured all three books in about as many days.
Tamora Pierce - I'd recommend Song of the Lioness, as this is the quartet I read, but the girls to whom I gave my books once I'd outgrown them have read everything they can of hers, and still love it - good for girls looking for a fighter in their stories, as well as passion, strength, magic. Perfect for 11-14 year old girls.
For younger children - Alexis Deacon's 'Beegu' is a beautiful story - I'd buy it for anyone, of any age, and bought it for myself despite it being a picture book. Would be perfect for getting older kids to read to those younger, or for when you're trying to get them to write and draw their own stories.
Shaun Tan's 'The Red Tree' is a book about depression, but it's a picture book. Again, I'd recommend it for readers of any age. This book should have won every award going - for the illustration, the Greenaway award, for the content, the Orange, Smarties, and Blue Peter. I don't know why it didn't, but it's a genuinely beautiful book - a keepsake and a future classic.
Shirley Hughes' 'Ella's Big Chance' recently won the Kate Greenaway award for illustration - this is not only the best example to date of the perfection of the books this writer of Dogger and Alfie & Rosie has produced, but also a beautifully un-Disney retelling of Cinderella