.controlOff{
display:inline-block;
width:130px;
height:36px;
cursor:pointer;
background-image:url("../iclass/images/teach_off.png");
background-repeat: no-repeat;
}

.controlOn{
display:inline-block;
width:130px;
height:36px;
cursor:pointer;
background-image:url("../iclass/images/teach_on.png");
background-repeat: no-repeat;
}

<a href='javascript:void(0)' id='teachControl' class='controlOff'></a>

function switchTeachControl(){
var target = $("#teachControl");
if(target.hasClass("controlOff")){
target.removeClass("controlOff");
target.addClass("controlOn");

}else{
target.removeClass("controlOn");
target.addClass("controlOff");

}
}