On all driving school cars you will see a 'certificate' displayed near to the car tax disc.
A GREEN badge defines that this driving instructor is FULLY QUALIFIED and has passed all three DSA qualifying examinations to become an ADI (Approved driving instructor). They are very tough examinations and only around 30% who start the training course actually qualify. See the green badge in the photograph of our car - it's just above the car tax disc. This is where every driving instructor MUST display their badge.
You will sometimes see a RED badge in the same location on some driving school cars. This shows that the instructor is a TRAINEE and has not yet passed the final qualifying examination which tests the instructor's teaching ability.
Many driving schools allow trainees to teach their pupils in an attempt to offer cheap driving lessons.
Do you HONESTLY want to be taught by a trainee? You could even be the instructor's first ever pupil. You would not know.
So always check the colour of the instructor's badge on your first driving lesson.