你是想表达：在你们班级,女孩是男孩的两倍是吧.但是你上面的表达都没有谓语,这应该是个完整的句子才对.
有以下表达：
in my class,girls are two times（or twice) as many as boys.
in my class,there are twice more girls than boys.
倍数表达：
1.times+形容词（副词）比较级+than
eg:This hall is five times bigger than our classroom.
2.times+as+形容词（或much)或副词原级+as...
eg:The big box is four times as heavy as the small one.
3.times+the+名词（size,height,weight,length,width)+of
eg:Your playground is 900 metres wide,three times the size of ours.