Complex sentence: It stars Jack Black, Joan Cusack, White, Sarah Silverman, Miranda Cosgrove and Joey Gaydos Jr. struggling rock singer and guitarist, Dewey Finn (Black), who is kicked out of his band and subsequently disguises as a substitute at a prestigious prep school.
Split into simpler sentences: It stars Jack Black, Joan Cusack, White, Sarah Silverman, Miranda Cosgrove and Joey Gaydos Jr. Struggling rock singer and guitarist, Dewey Finn (Black), who is kicked out of his band and subsequently disguises as a substitute at a prestigious prep school.