Change your code to:
<script type="text/javascript">
function sav() {
var class_id5 = document.getElementById('ddlClass5').value;
var class_id6 = document.getElementById('ddlClass6').value;
var class_id7 = document.getElementById('ddlClass7').value;
var subj_id = document.getElementById('ddlSubject').value;
var mark5 = document.getElementById('txtMark5').value;
var mark6 = document.getElementById('txtMark6').value;
var mark7 = document.getElementById('txtMark7').value;
$.ajax({
type: "POST",
url: "Student.aspx/SaveData",
data: "{class_id5:'" + class_id5 + "',subj_id:'" + subj_id + "',mark5:'" + mark5 + "',class_id6:'" + class_id6 + "',class_id7:'" + class_id7 + "',mark6:'" + mark6 + "',mark7:'" + mark7 + "'}",
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function () {
alert('success.');
}
});
}
</script>