Have you ever tried out the traffic light cup experiment when teaching maths to the younger years? It isn't reallly suitable for older kids who are too cynical, but ..
(If you are not familiar with it - each pupil has a stack of three paper cups on their desk where the teacher can see them, green, orange and red. Green on top means you are listening and understanding. As soon as they start to misunderstand what is being taught, you remove the green to see the orange cup. The teacher can then see when they are beginning to lose the class, and eventually as cups turn red you have to give that group some extra explanation. I've seen a maths teacher astonished as the whole class turned red within 5 mins, and she had no clue that they couldn;t follow her - it transformed the class.)
(Also if your girls think you have favourites, just use lolly sticks with names on and pick one out of a jar - no favourtism then.)