变量对象属性名称作为JavaScript中的参数？

我有如下的javascript对象：

var school = {
    grade1:[
    {
    "teacher":"anna",
    "student":"jacky"
    },
    {..},...
    ]
    grade2:[...]   
}

我尝试使用函数检索内容时遇到问题

function getNames(grade){
    $('#demo').text(school.grade[0].teacher); //returns an error
}

我可以使用，school.grade1[0].teacher但是我想要一个grade变量。

当我尝试这样做时，它显然不起作用：

var grade = 'grade1'; //I also tried just grade1 but it is an undefined varaible
getNames(grade);