The instant verdict is given by pupils at Highbury Fields school, Islington, and the City of London school for girls in the Barbican.
They have hand-held gadgets allowing them to show how well they have understood a lesson - yes, no or a little. Teachers can ask for a vote at any point in a lesson and decide whether to keep the result secret or share it with the class. They do not have to pass on the figures.
Depending on the result, they can change their teaching method, divide the class into groups or carry on.
The six-button keypads guarantee anonymity and are linked to interactive whiteboards. Staff have also used the devices for quizzes and to elect a head boy and girl.
The keypads are used in science, humanities and ICT at the schools, which are trialling them as part of a stateindependent school partnership. Each has 32 machines, costing pound;5,000. Ian Phillips, director of ICT at the City of London girls, said: "We have only had them a few weeks but I have used them quite a lot. I plan when I am going to use them at strategic points to see if the lesson is working."
Florence Chamberlain, a 16-year-old at City of London girls, said: "Before, if I didn't understand something I would switch off and my mind would wander.
"But now if I don't understand, then I have the chance to say so. It makes everyone more involved in the lesson and more likely to concentrate harder as we know we will be asked if we understand. No one is embarrassed if they don't understand. It is good that we can voice our opinions and means we are to learn more."
Tomas O'Donnell, head of science at Highbury Fields, said: "If you asked pupils to put their hands up if they understood, they would first look around to see what their friends were doing before answering. This way you get a clear indication of whether the lesson is working or not."