They were all listed as such in the credits. Most of the background students (beyond the main characters, and people like Dean, Neville etc) were cast as extras and then given names of characters from the books in the credits. This is why some extras disappear between movies (the red headed, vacant looking girl who plays 'Susan Bones' from the first two movies disappears for the third), and Lavender had three different actresses, as two were cast as extras and the 6th movie required an actual speaking/acting role.

http://harrypotter.wikia.com/wiki/Lavender_Brown HP wiki on her. See the notes at the bottom