''I did a lot more yelling on this movie than I normally do,'' says director Malcolm D. Lee of trying to keep a cast of hormonally charged young men in line long enough to wrap principal photography on his '70s-set roller-boogie romp. ''It's like Bill Cosby said years ago they've got brain damage.'' It's fitting that Lee would quote the Coz, since his tale of an Afroed teen struggling to balance dating, skating, and a shaky relationship with his widower dad (Chi McBride) during a life-changing Chicago summer is just the sort of warmhearted comedy the Huxtable patriarch could get behind. But portraying the era's pre-'80s styles meant halting hip-hop heartthrob Bow Wow's swagger. ''I told him, 'You can't be you,''' says Lee. ''The minute we get taken out of the movie by 'Ya know what I'm sayin'?' or 'Aight!' we're lost.''