You guidance for this is in the IRM guidelines which state that paper files for pupils with SEN must be kept by their last school (which is usually their secondary) until they are 25 + 6 years. (25 years for the life of any possible EHCP and 6 for any data retention and possible retrieval requests). https://irms.org.uk/page/SchoolsToolkit
If you are a primary school then all the records should have been passed on to the relevant secondary school. The IRMS Toolkit has guidance on how to securely destroy information too.