"our emphasis on collaborative, active learning tends to encourage it."

In my childhood experience, 'collaboration' means that one person does all the work while the others screw around. Since no outside pressure is exerted to ensure that all parties contribute, this just amplifies existing social biases. If you put the 'cheerleader' with the 'nerd' and don't check in to make sure they're both working, all you did was hinder the 'nerd'.