In the film / DVD "Romy & Michele's High School reunion" during the opening credits Gwen Stefani sings her song "I'm just a girl.." Having bought the soundtrack years ago, I wonder why this song was not included in the tracks on the CD.

As you probably know when music appears in movies or on soundtracks there are royalties to be paid.  Often there ends up being more songs in a movie then there are on the soundtrack.  Occasionally more than one soundtrack for a movie will be released in order to accommodate all these songs.  Most often however, only one CD is made and the decision of what songs to put on that soundtrack are decided by producers who have a budget to work with.  While Just a Girl by Gwen Stefani is a good song, it is also a popular song and the producers probably decided that it didn’t make the cut for the CD due to how much that would have cost them.  They would have had to pay royalties to have it in the movie at all, but they would have had to pay more to put it on the CD.