Hiya! I had my interview last year and I am now an Upper Sixth student at Kimbolton.
Be prepared to discuss your chosen subjects and why you're interested in them. Any work you’ve done that goes beyond your subject will be especially interesting. This doesn’t have to be especially refined - documentaries, books, films will be very impressive. For instance, I was applying to study an a-level in English Literature, so I was asked what I am currently reading. Familiarise yourself with the sixth form's curriculum, and any special programs or extra-curricular activities they may offer or initiatives related to your subjects that you might like to take part in.
Be ready to expand on the points you made in your application. Reflect on your achievements, experiences, and how they have shaped your academic goals. Discuss any academic achievements, extracurricular activities, or responsibilities you've taken on. Discuss how these experiences have contributed to your personal and academic development.
Maintain a positive attitude throughout the interview. Express enthusiasm for the subjects you've chosen and the prospect of studying at the sixth form. Make sure to focus on extra-curricular activities you might enjoy.
They may also ask you more general interview questions (e.g. describe yourself in three words.) Focus here on academic descriptors - (punctual, diligent, etc).
Best of luck! I hope this helps.