The FAQ on parental involvement is pretty informative:
http://www2.ed.gov/programs/titleiparta/parentinvguid.doc

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E-5.​Is volunteering in a child’s classroom an activity in which parents can engage to help share the responsibility for student learning?

Yes. Volunteering and observing in their child’s classroom is an important activity for parents’ shared responsibility for high student academic achievement and is also one that helps both the school and parents build and develop a partnership to help children achieve the State’s high standards. Section 1118(d)(1), ESEA.