Question

高中生活和初中生活完全不一样用英语怎么说

Answer 1

High school life is different from that in junior school at all.
这里的that是指代生活，因为上文中提到了高中的生活所以这里直接用它来指代，避免重复。“完全不同”就用at all 表示，就是“根本不，一点也不”

Answer 2

The life of High school is quite other than Middle school.
The ilfe of high school is completely different from middle school.