Marlboro High School which is one of Freehold Regional HS District's high schools. However students can apply to special programs in any of the six High Schools and be transported to that school should they be accepted: Marlboro, Freehold TWP, Freehold Boro, Manalapan, Howell, and Colts Neck. All are excellent schools and a good reason to move to the area.
Keep in mind there are a few streets in Marlboro that are actually zoned for Colts Neck High School. Although, I don't think those streets are within the Morganville section. See the street guide below from the FRHSD website.
Morganville is actually a section of Marlboro. As such, they would attend Marlboro High School. Unless, they considered applying to one of the Learning Academies through the Freehold Regional High School District or to one of the Monmouth Country Vocational School District Schools. See my blog post on the options available to High School students in our area.