You should check with your insurance company to see if your son is required to be placed on the policy or not with a school permit. Insurance company guidelines can differ. Some insurance carriers will want your son added now, while others may allow you to wait until your son has a full driver's license.
Usually your policy terms require you to notify your insurer of any newly licensed household members so advise your insurance agent about your son's licensing status and see when they say that you must add him to the policy.
In Nebraska a School Permit (SCP) authorizes the holder to drive a motor vehicle, moped or motorcycle to and from where he or she attends school by the most direct and accessible route by the nearest highway from his or her place of residence if such person lives a distance of one and half miles or more from such school and either resides outside a city of the metropolitan, primary or first class (5,000 or more people) or attends a school which is outside a city of the metropolitan, primary, or first class (5,000 or more people).
A school permit shall be used for the purpose of transporting such person or any family member who resides with such person to attend duly scheduled courses of instructions and extracurricular or school related activities at the school, and the holder of the permit may drive under the personal supervision of a licensed driver who is at least 21 years old. To obtain a SCP one must have held a learner's permit (LPE) for 2 months prior to applying for SCP and be a minimum age of 14 years and 2 months.