It is the student that/who makes me happy.这个是正确的说法。

如果说是指人的话，就可以用who，当然，that,是通用的。改成students之后，就用应该it is the students that make me happy。再强调句型中的要注意的就是还原，还原回一般句子，譬如说你这句话的一般句应该是the student makes me happy.那你现在要强调的是主语，所以只要把主语提前到句型的it is 后面。当你变成复数时，就意味着原来的句子是 students make me happy.所以强调主语时候变为it is the students who make me happy.