From the National Curriculum Key Stage 2 P.E.
During the key stage, pupils should be taught the Knowledge, skills and understanding through five areas of activity:
dance activities
games activities
gymnastic activities
and two activity areas from:
swimming activities and water safety
athletic activities
outdoor and adventurous activities.
Swimming activities and water safety must be chosen as one of these areas of activity unless pupils have completed the full key stage 2 teaching requirements in relation to swimming activities and water safety during key stage 1.
So they HAVE to do swimming in Key Stage 2 unless they can already do the following by the end of Key Stage 1
pace themselves in floating and swimming challenges related to speed, distance and personal survival
swim unaided for a sustained period of time over a distance of at least 25m
use recognised arm and leg actions, lying on their front and back
use a range of recognised strokes and personal survival skills [for example, front crawl, back crawl, breaststroke, sculling, floating and surface diving].