Pupils inclined toward humanism, or old enough to call themselves humanists already, and who happen to have explicitly humanist parents, often get enough support in the free development of their world view and moral values from their families, limiting the damage than can be caused by exclusory and presumptuous RE. Such pupils with less family support, for example whose parents are not themselves very interested in such matters and don't profess a cohesive "world view" themselves, may find little support anywhere, and may grow to think that not only religion, but morality too, is of no personal significance.
(Collective worship in schools compounds the above problems, and causes some of its own. For example, whether to join in or not poses problems for the conscientious humanist, and, in some schools, a refusal to pretend to pray meets with reprimands.)
Source: here