If they do it right, any student should be qualified to teach any other student in a lower grade. That's how it always was before them rockefeller types bought up the whole education system and industrialized it.
And it's not only a fine way of teaching, but also an excellent way to solidify what the teaching-students have already learned.
Which is why American education exploded into excellence, but then when 'ol Rocky decided to make every kid into a "factory worker" it suddenly imploded into ignorance.