Zac Efron, of the popular High School Musical TV-movie, lends his voice this Saturday to The Replacements, Disney Channel's animated series about tween siblings who order supercool parents through a comic-book ad. Efron will voice Davey Hunkerfoff, a (you guessed it) hunky new lifeguard at the town pool. In its first month on the air, The Replacements (Saturdays at 8 pm/ET) has ranked No. 1 among broadcast and basic-cable series with kids age 6 to 11.