Final answer:
The best initial action for a school nurse when a child has a seizure is to stay with them and call for emergency medical services while offering calm reassurance.
Step-by-step explanation:
The most appropriate initial action by the school nurse when a 10-year-old child experiences a tonic-clonic seizure e for the first time is to stay with the child and have someone call emergency medical service (EMS). This response ensures that the child is not left unattended during a potentially dangerous event and that professional medical assistance is on the way. It is also paramount to offer calm reassurance to the child during the postictal state, the period after the seizure characterized by potential confusion and disorientation.