The question was how do you track whether people have opted in or not, by not having biometrics for people who have opted out. The solution is simple, if you do not recognise the person, they have not opted in.
According to the article, the experiment was constructed as opt in and several students have opted in.
The thing is, aren't all the efficiency gains wasted if even just a couple students opt out? You've still got to take a register, even if shorter. It can't be worth it for a few extra minutes.
