Dawson Casting: Regina George is played by Rachel McAdams, who was 24-25 at the time of filming. Also, Lacey Chabert and Lizzy Caplan were both 21 when the film was made. Lindsay Lohan and Amanda Seyfried, however, were both teenagers at the time.
A lot of the actresses in the sequel have been featured on Disney Channel at one point (Save for Claire Holt, who was in H2O - Just add water which aired on Nick and TeenNick in the US, but averted in Australia since her show was aired on Disney Channel Australia). That fact doesn't really help matters.
In the original script Gretchen was envisaged as plain-to-ugly (with a "sniveling whiny face") and part of the in-crowd solely for her wealth. Obviously this was dropped when the very pretty Lacey Chabert was cast in the role but traces of the original perception of the character can still be seen in the film in Regina's obvious surprise that Gretchen was nominated for Spring Fling Queen.
The film was a lot filthier before it was PG-13ified. See Bowdlerization on the main page.