As MurphysMom said, if you are in the city, check out Masterman. My eldest knows several kids there, and it is clearly the best public school in Philadelphia.
I don't know where you are exactly and your situation, but there are - as MurphysMom explained - some very good public schools near Philadelphia as well as a ton of private schools. If moving to Lower Merion, Radnor or T/E is possible, then public school is an option (there are other good districts - try to find the Philadelphia Inquirer yearly article on area schools). Private schools vary in culture and academic rigor. Try to find the Philadelphia magazine article on area private schools that was published a few months ago. While I'm sure you know about some of the private schools, you can check out a chart to see which ones have the highest SAT scores:
http://www.phillymag.com/articles/private-school-guide-2012/Of course, not every kid in a private school is going to be HG/PG, but there are a lot of bright kids. As for the public schools, there are plenty of bright kids there too - in T/E, 7% of the current senior class is NMSF and it is 8% of the class in Radnor.
For the HG/PG kid the classes might still be easy at any school, but at least they will have intellectual peers in many of the private schools and at the public schools mentioned above.
