In class the teacher is in charge. He might not teach everybody, but he should oversee everything that is going on in the class. In the past teachers often did not have that many students, sometimes only one or two, so it was very easy for the teacher to teach them. He or she would find the time to observe the students and when they were good enough would teach them more. In class there can be a lot of people and so one teacher cannot teach everybody, especially if everybody is at a different level and so the senior students will help out.
When a senior student is appointed to help a junior or beginner, they are in fact being given an honour and they should not think that they are doing the teacher a favour. The teacher is trusting them and by allowing them to help out means the teacher thinks that their skill and knowledge is growing. On the other hand some senior students are never asked to help beginners and this is because they are not ready, perhaps their level is not good enough to teach at that moment.