Challenging every child in the classroom is a difficult task, and if exercises are not presented at the right level, then decline in progress and enthusiasm are likely to follow.
Students can be easily discouraged if given inappropriate problems beyond their understanding, while others often linger if faced with too little challenge. Problems must be presented at the level appropriate to the student's skill set and age.
Many children are needlessly lost in the great jungle of mathematics: they either 'don't get it', or find it 'trivial and boring'. The most important task of a teacher is to get students excited about what they are learning, so that they want to know more, and they make great progress without ever thinking that learning was hard work.
Mathematics is not something to be frightened of, and the private lessons aim to foster enjoyment and boost confidence in the subject. The student may even face tougher challenges than in the classroom and likely to gain valuable mathematical insight.