Perhaps a bit OTT. If it is a Summer Ball, you'll be running around, on dodgems, all sorts of activities, so something that long would cause you real problems. You would also need something to cover your shoulders with whilst eating - a shawl, wrap, jacket etc. A split to the top of the thigh might also be a bit excessive. Your boyfriend should get proper advice so that you don't turn up improperly dressed.
More than anything else, make sure the dress suits you and fits you properly.