Not a bad idea as long as your daughter is willing. If she is going to drag her feet and not want to go she won't get a lot out of it IMO. Would she be in the same class as her sister? Would the two of them taking it together make it more fun or less? One of the things I've learned from this board is that gifted kids often can use study skill reinforcement. There often is a big difference between elementary and junior high. Particularly the change in having multiple teachers with different deadlines.
