That makes sense. Opt out for even just one individual would be effectively impossible, otherwise how would the system know who's opted out without remembering them in some way.

This was a tech trial where 22 students and their parents (both separately) opted in to take part over 3 weeks. The teacher also opted-in.

A person who has not opted in has opted out. No tracking necessary.

You really think they're going to design things opt in and not opt out?

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.

It's covered by recital 43 which deals with public institutions and power imbalance.

There is no practical way for a student (or their parents) to say no, given the circumstances.