Jack Beechwhistle: Attack of the Giant Slugs by Kes Gray
Don't panic, but you're in danger. At least, you would be, if Jack Beechwhistle wasn't around to protect the world from alien attacks, zombie sweet-shop owners, and exploding conkers - with a bit of help from his trusty sidekicks, Colin and Harry (as long as it's not past Colin and Harry's bedtime). Their missions are daring, dangerous, and deadly. They even have top-secret headquarters (the shed by the rhubarb patch). But now Jack's about to face his most challenging challenge yet. The Attack of the Giant Slugs. And they're on their way to Jack's school - right in the middle of maths!