Please note that it is not a GPs responsibility to issue school letters of absence for your children. Government guidance is that school attendance is mandatory unless your child is self-isolating, has symptoms of COVID-19 or is close to someone who has COVID-19.’
‘The Government has stated that from September school attendance will be mandatory once again. This decision has been taken because the current level of risk of infection is low and therefore “perceived risk of COVID to child or parent” will not be considered adequate grounds for a child to not attend school’ https://www.gov.uk/government/news/schools-and-colleges-to-reopen-in-full-in-september
Please note that parents should still make arrangements to contact their GP in the normal way if their child is ill.