Any ones you want! The contest is adaptive, so please don't feel that only the brightest students can participate. Each student will be given questions for their level, and class results are based on accuracy not difficulty.
You will need to have a minimum of 10 students in a class for it to appear on the leaderboard, and be eligible for prizes, but we do not have any maximum limit.
There is no age limit for students taking part. We generally cover numeracy topics for ages 6-13, but this may depend on the ability level of the student. Special schools, for example, will have students at varying levels which may or may not be related to their age.