Plenty of research shows that when people use an avatar that, for whatever reason, they deem desirable, they act in a more confident way - sometimes for the rest of that day.
With the launch of Hwb+, children will be designing an avatar to use, probably using a site like DoppelMe or PlanetCreation.
So... let them be creative, let them create an avatar that they are proud of and then use it outside of Hwb+. Display it in the classroom, put it on their drawer, learning diary or desk. Make sure that they will engage with that avatar daily and enjoy whatever benefits come from having a class of more confident learners.
You'll have no way to quantify the effect, but you'll be one step closer to a games-rich classroom.