Seeking Medical Support
Your GP is your first point of call to discuss your concerns. Be sure to discuss the true impact of the anxiety on getting your child to school. Make sure that the GP is fully aware of the amount of time being missed from school. Make sure you are aware of the true extent of absence (odd days all add up and parents are often surprised at the true level of absence themselves).
If the anxiety begins to really impact regular attendance at school or if it occurs on top of other psychological concerns (self harm, depression, suicidal threats or eating concerns etc) then ask your GP for a referral to Camhs (Child & Adolescent Mental Health Services). Alternatively, the school may have good links with the local School Nursing Team who may be able to make this referral on your behalf.
Do not delay in seeking help, delay can have a significant impact on getting your child reintegrated back into school or prevent an entrenched habit of avoidance. Remember, avoiding school can become a difficult habit to break.
Before you visit the GP, really consider the impact that the anxiety is having on your child and their life. Go armed to the GP with a list of the ways the anxiety is impacting their life (school, eating, sleeping, socialising, isolating themselves, low moods etc). Get it all on paper and go prepared. If the GP is unhelpful, you may need to persist.