Tutoring and conducting special classes are notable exceptions but these are not what most volunteerism is about. Apart from these exceptions, volunteerism as parental involvement is erroneous. Parental involvement is the carrying out of defined responsibilities that have been worked out between parent and teacher in support of student performance. It has nothing to do with administration or with volunteerism. Parents who are involved with their childrens' learning may donate time to the school but that is volunteerism, not parental involvement.
Parent involvement has to do with the special and unique relationship between parent and child.
It is not charity. In the unequal relationship between teacher and administrator, the stronger member frames the issues. Administrators have defined parental involvement in a way that serves their own parochial interests. The practical solution is to divest school administrators of curricula responsibility so there can be no pretense of volunteerism serving these ends. Teachers must be given sovereignty over their classes and parents must be made co-equal partners with teachers so they can work together unfettered.