When you set up your account you will be asked to create a unique link for your school's Theory Test Pro website. You'll also be provided with a secret word.
You can change your secret word to something more memorable that would be easy for students of your school to know but difficult for outsiders to guess - for example: what is the last name of the teacher in charge of sixth form?
Students need to sign up for an account at your school's Theory Test Pro website address. They will be prompted for your school's secret word when they sign up and every time they sign in.