First, make sure...
...That it really is a ringworm
; many circular rashes are assumed to be ringworm by inexperienced caregivers.
If it really is ringworm, once treatment is started, it is okay for your child to go to school. Treatment depends on the location; if it's on the face or scalp, make sure your doctor gives you an oral treatment - creams and lotions won't get the job done in most cases.