应该是Classmate party。
因为名词+名词=名词短语,且该名词的数应保持一致。在书面语中很少见到classmates party,有的只是classmates ’party,是名词所有格的形式。
同学聚会的表达还有:class igeunon;classmates' party;classmate party;reunion part;class igeunon。
classmate
n.(同班)同学;级友;
复数:classmates
例句
1、Because by letting a classmate read your draft.
因为通过让同学读你的稿子。
2、Supreme court justice clarence thomas was a classmate too but Inever got to know him.
最高法院法官克拉伦斯.托马斯也是我的同班同学,但我一直没有机会与他结识。