Complex sentence: Students in grades 9-12 attend the Mainland Regional High School, which is the single high school of the Mainland Regional High School District, and which serves students from Linwood, Northfield and Somers Point.
Split into simpler sentences: Students in grades 9-12 attend the Mainland Regional High School, which is the single high school of the Mainland Regional High School District. The district and high school serve students from Linwood, Northfield and Somers Point.