I second the Mainline. Not that I am located there
... First of all, it's a lovely area - quite pretty. They have great summer camps up there at the colleges. I work up there currently and am sending my sons to some summer camps this summer at Villinova and Rosemont. And I 'hear' great things about Radnor SD. I know a guy who was fully homeschooled in the Radnor district (very wealthy family and he was homeschooled via private tutors) but he participated in all of the extracurriculars and sports at the school. It is a rather wealthy area.
Unionville-Chaddsford District is suppose to be good too. Again, no real personal experience except that DS does chess club there. I do have a friend whose daughter is there and last I heard her daughter was bored to tears but no g/t until 3rd. I am not sure how they ID gifted but she didn't say her daughter got in in third grade.
