I read it in maybe 2004 or something from my primary school library. The cover looked like it'd be from the 80's or 90's or early 00's but I'm guessing the 90's (at least that print anyway). About the size of a goosebumps book.
A new family comes to town, the 'Valentine's' and I'm pretty sure the girls name was Amy, and she wore a heart locket necklace. There was something strange about her and her family. She invited her whole class to a halloween party at her place, which I'm pretty sure was a mansion.
The class thought her house was so cool, and they loved her parents who were dressed up too and tried hard to be good party hosts and all the kids were having a great time. But the protagonist, or one of them, I can't remember, I think it was a guy, he threw his drink in the potplant either because he thought it looked or tasted weird or he wasn't thirsty and he was the only one who didn't drink it I think. Throughout the night kids started disappearing and they realised something was up, so a group of them tried to escape. Only one of them (this guy who didn't drink the drink) got away, and the next day he came to school, everyone was there and he was surprised, but then realised there was something very wrong and off about everyone and they all had a glassy eyed stare or something.
Also I feel like Amy Valentine had a weird thing she said at the beginning and end of the story, but I can't remember what it was. But anyway, her parents turned all the kids into something weird like them.