Yea I agree with the last poster for the most part. Recent West Potomac grad here. I know people that went to WP, TC Williams, Bishop Ireton (Catholic school), and Mount Vernon. Honestly, if your kid has a good head on his or her shoulders, he or she will be fine WHEREVER they go.
As for the difference between a Langley and a WP, Fairfax County is so highly ranked for a reason. Their are great teachers at every single school in FFX County. I can't speak as much for BI or TC, but I know TC is really improving, and, again, your student would be fine there. I've gone through 2 different pyramids in the FFX County system, both were great.
Sure, there are drugs (and alcohol, but we all start drinking at some point) and gangs and fights at every school, but who cares? WP apparently had more fights than any other school in the county when I was there, and I witnessed, oh, i dunno 3 fights in a year at most? It's nothing to worry about.
There will be a bad teacher at every school, but as I tell people, that is only going to make you stronger as a student. And with strong PTA's and communication with parents at all FFX county schools, if there is a REAL serious issue, it will be dealt with. I had one "serious issue" in my 4 years: While I personally had my issues with the teacher, I survived the class just fine. Some of the, um, more spoiled and pampered students just felt the need to complain, because she was expecting a bit too much out of them or something, and not really teaching very well. I don't remember the exact reasons, but at the end of the day, as a student, you need to be independent, and as long as you are, you'll be totally fine!
Your son or daughter will find a great group of friends and that will really be important to their success and development as well.