Off the Bookshelf: UNITED WE SPY, by Ally Carter
The Gallagher Girls series. The (sob!) last one.

Heart-racing adventure indeed. Races against the clock -- and races that the gang sometimes lose, to great heartache. Enemies become surprising allies, and allies become enemies, and sometimes that isn't surprising and it all comes down to a literally explosive ending that ... well, I don't want to spoil it for you. Although if you love the Gallagher Girls like I do, even if you KNOW how it ends, or doesn't end, and who comes in from the cold and who ends up cold .... and shredded ... you'll still read it!
Oh, by the way, did I mention that the big threat is the nasties attempting to trigger WWIII?
Yeah, typical stuff for the girls of the Gallagher Academy. Because when you underestimate your opponent, you always lose, and everyone underestimates those private school girls in their plaid skirts........ BIG mistake.