There needs to be at least 10 students in your class for it to be eligible for entry. This is to make it fair for big classes as the score is based on the class average.
If your class is too small, you may wish to team up with another class. To do this, you could create a new class or move students into your class just for during the contest week. Because students will answer questions targeted to their abilities, you don't need to worry about your contest team being comprised of mixed ages.
We do this to prevent cheating and to make our contests as fair as possible.