Girl, Going on 17: Pants on Fire by Sue Limb.
If you have not read the first book in this series: Girl, 15, Charming but Insane, you really must!   Sue Limb is a British author who manages to capture the horror and joy of being a teenager with hysterical humor. 
Now that Jess is 17 years old and happily going out with her perfect Fred, what could go wrong?  Well, how about having a really awful row with Fred right before school starts, discovering that her favorite teacher has been replaced for the term with an utter hag, and finding herself in the tiniest bit of trouble wearing a short skirt and no underwear.  This is perfect British chick lit for teens.  I would compare it to Bridget Jones, but I find Limb’s writing deadly funny and right on the mark almost all the time.  I was glad to find that Limb had returned to a school setting for this third book in the series.  Though the second was funny, this one returns to the glory of the first book. 
Great fun fluff to recommend to any Anglophile teen and even those who aren’t.  This book is the perfect humorous counterpoint to all of the horror and angst found in most teen books.