From the Key Stage 1 Assessment and Reporting Arrangements (sets out annual legal requirements for heads):
*Headteachers should:
? ensure their teachers fully comply with all aspects of the Key Stage 1 assessment,
moderation and reporting arrangements;
? identify the children to be assessed at the end of Key Stage 1;
? ensure all task and test materials are stored responsibly and that children do not
have access to the materials before they are used;
? ensure Key Stage 1 tasks and tests are administered;
? ensure children?s responses to the tasks and tests are marked accurately and
consistently;
? ensure teacher assessment levels for reading, writing, speaking and listening,
mathematics and science are submitted to the local authority or other recognised
body for all children;
? ensure teacher assessment levels for reading, writing, speaking and listening,
mathematics and science are recorded for all children at the end of Key Stage 1;
? ensure teacher judgements are monitored;
? provide teacher assessment results to their school?s governing body to enable it to
comply with national data submission requirements and report to parents;
? provide parents with written reports on their child?s progress free of charge, and
provide the opportunity for discussion at least once in each school year;
? ensure the statutory requirements for the transfer of records between schools are
fulfilled, including the completion of the common transfer file;
Academies are also legally required to carry out the assessment.
I know that most schools make this as low key as possible for these young children - after all, it's run of the mill teacher assessment as in any other year, except that a small part of the process is a test (for those working at level 2 or above) and the results are reported to parents, the LEA and the DfE, but you would think that another teacher might have noticed 
Joking aside, I think it's pretty irresponsible for a teacher to suggest on a public forum that this is in any way optional, Jayne, and that a school may 'choose', especially since the OP asked specifically.