While I could simply refer her to my "Top 45" list of books to get hooked on after Harry Potter, I reckoned she wanted a somewhat narrower scope. Nevertheless, I couldn't just give her one choice and say, "Read this, this is the closest you can get to Harry Potter." Instead, I came up with a dozen series of books, each similar to Harry Potter in some way. If you're where Tracy's at, perhaps my response to her can help you too. Here's what I wrote back:
I'm not sure which one you would consider closest - depends on in what ways you want it to be similar to Harry Potter. Nothing is exactly like it, but at least a dozen series come to mind. Each in its own way is "close" to Harry Potter. Try them and let me know which one you like best.
- Diane Duane's "Young Wizards" series, starting with So You Want to Be a Wizard - New York City teens learn wizardry together at home. Excellent quality stuff, IMHO.
- Emily Drake's "Magickers" series starting with (duh) The Magickers - American kids learn magic at summer camp. Not as well done as Harry Potter, though.
- Diana Wynne Jones' "Chrestomanci" series, starting with Charmed Life - British kids in an alternate universe learn magic via private tutoring in a "Gosford Park"-style setting. Very good.
- E. Rose Sabin's trilogy beginning with A School for Sorcery - I have only read the first two books, but Book 1 takes place in a private school for the magically talented, and Book 2 is a prequel that explains how the school was started.
- Caroline Stevermer's trilogy beginning with A College of Magics - only the second book and half of the first book take place in a school of magic, and it's more of a college-level setting.
- Ursula K. LeGuin's A Wizard of Earthsea is partly set at a school for wizards. This is the first book in a series.
- Suzanne Collins's "Underland" series, starting with Gregor the Overlander, doesn't have a school of magic, but it does share Harry Potter's theme of a boy who enters a strange, undiscovered world and finds out that there are prophecies about him.
- David Lubar's Hidden Talents and its sequel True Talents are about a group of misfit kids who meet at a correctional school and discover their "super powers" together.
- Jonathan Stroud's "Bartimaeus" trilogy, starting with The Amulet of Samarkand, is about an apprentice wizard who rebels against his master and goes on to change the magical world.
- P. B. Kerr's "Children of the Lamp" series, starting with The Akhenaten Adventure, is about young djinn (genies) discovering their talents and the nature of the magical world they belong to - though they don't actually go to a school for djinn.
- Rick Riordan's "Percy Jackson & the Olympians" series, starting with The Lightning Thief, concerns a present-day summer camp for mortal children of the Greek gods.
- Cinda Williams Chima's trilogy starting with The Warrior Heir also depicts teens finding out they have magical powers, including wizardry, and book 2 (The Wizard Heir) is partly set in a really scary school of magic.
EDIT: Oops - I just remembered another series for you, which makes a "baker's dozen"... Jenny Nimmo's "Children of the Red King" series, starting with Midnight for Charlie Bone. This series features a sinister boarding school where magically talented kids get extra attention. It's aimed at the younger end of the age range for Harry Potter fans.