I don't believe that the year 11 interview will have a group stage as the interviews are set 15 minutes apart. The interviewers really just want to see if you're coming to JMSS because it's a selective school or if you are genuinely passionate about sciences. The best advice for this interview would really be to show your personality to the interviewer. Show them that you are open minded and are confident in your abilities and skills. Prove that you would be a good fit at their school. Do not be a know it all, instead reflect on yourself with both strengths and weaknesses, showing that you are putting effort into fixing those weaknesses. Good luck!