Usher:
You better watch out
You better not cry
Mandy:
You better not pout
I'm telling you why

(You know that)

Both:
Santa claus is comin' to town
Santa claus is comin' to town
Santa claus is comin' to town

Mandy:
He's making a list
Usher:
He's checking it twice
Mandy:
He's gonna find out
Who's naughty or nice

(You know that)

Both:
Santa claus is comin' to town
Santa claus is comin' to town
Santa claus is comin' to town

Mandy:
He sees you when you're sleeping
He knows when you're awake
Usher:
He knows if you've been bad or good
So be good for goodness sake

Usher:
So you better watch out
You better not cry
Mandy:
You better not pout
I'm telling you why

(You know that)

Both:
Santa claus is comin' to town
Santa claus is comin' to town
Santa claus is comin' to town (x3)
[ Lyrics from: http://www.lyricsfreak.com/m/mandy+moore/santa+claus+is+comin+to+town_10172962.html ]