Is this Teen Sorcery, from 1999?
Details which fit:
- New girl in school
- Magic necklace/amulet
- Antagonist (played by Lexa Doig) is a brunette in the film (and could be probably be mistaken for Rachel Leigh Cook in a fuzzy memory)
- Time frame is correct
Full movie seems be available for streaming in lots of places, I found it on The Archive.