Aii of them are students.(改为否定句）

None of them are students.
None of them is students.

Both are OK!

None of them is student.

None of them are students

All of them are not students

None 0f them is students
none 谓语动词用单数

Aii of them are not students。
None 0f them is student

这位小同学，改否定句就是把“是”改为“不是”就行了。
以汉语为例：我是小学生---- 我不是小学生

None of them is student。是完全否定，意思是他们之中没有一个是学生。
All of them are not students 。是部分否定，意思是他们不全是学生。