Jack Black is gearing up to celebrate a major milestone. ET's Matt Cohen recently spoke with the 53-year-old actor, who revealed his big plans to mark 's 20th anniversary this September.Black starred in the beloved film as substitute music teacher Dewey Finn, with young actors including Miranda Cosgrove playing his students.«All those kids — dig this — they were 10 years old when we made that movie and now they're all, like, 30,» Black marveled.