In the scene where Bronide and Renee 'make whoopie' in the elevator. Both of then go into the elevator wearing jackets. Brodie even makes a comment about 'Is that my jacket?' When they both walk out, Brodie is still wearing his jacket but Renee's is gone.
If you watch the movie, you will notice that in every scene Renee is in she wears a new outfit. Not only does she lose the jacket (Degrassi High jacket - Go Canada!!) after "whoopie" with Brodie, she is actually wearing a completely new dress entirely.
The reason Renee wore different dresses through out the movie is because Shannon Doherty got to choose her outfit for the movie, and chose all of them.