*SPOILERS IN THIS REVIEW*
I'm 18, I first read this when I was 15. My sister is 14 and just read it. This book is beautifully written with very realistic characters. Pudge, the main character, idolizes Alaska but it's made very clear in the book that she and the other characters are far from perfect. It discusses real issues, it has very realistic characters, and it has very important messages.
There's a lot of smoking and drinking in the book but no other drugs (some other review mentioned weed, it's mentioned maybe twice and to my recollection never used by the main characters) but these aren't in a negative influence way. At the start of the book, Pudge has never smoked, but he feels pressured into it and becomes addicted anyways, and Alaska literally says "I smoke to die" - it shows that it's bad for you, it doesn't glamorize it. Same with the alcohol to an extent. Although for most of the book it's just teens getting drunk to have fun, the book also focuses on the poor decision making that happens when your drunk, and (HEY MAJOR SPOILER HERE) a main character dies driving drunk. It shows that there are consequences to these things.
Also, it's not like your kids don't already know about cigarettes and alcohol. Don't try to shelter them, they're exposed to it during high school regardless of whether or not they read this book, and this book might help them make better decisions about drinking. (If you REALLY think your kids can go through high school without knowing about underage drinking, you're an idiot, and your kids probably already drink.)
As for the sex scene, yeah, it's there. But again, it's realistic (sorry parents, it is) and it serves and important purpose for characterization.
One really important thing to know though is (SPOILER) this book deals with suicide. If that could be triggering to your kid, possibly direct them away from this book.
Also, it can be a lil fake deep at times. But we all need a lil fake deep.