don't request another ajax call if one is pending (#9620)

erpnext/schools/doctype/student_attendance_tool/student_attendance_tool.js

@@ -135,6 +135,7 @@ schools.StudentsEditor = Class.extend({
 				frappe.confirm(__("Do you want to update attendance?<br>Present: {0}\
 					<br>Absent: {1}", [students_present.length, students_absent.length]),
 					function() {	//ifyes
+						if(!frappe.request.ajax_count) {
 							frappe.call({
 								method: "erpnext.schools.api.mark_attendance",
 								freeze: true,
@@ -151,6 +152,7 @@ schools.StudentsEditor = Class.extend({
 									frm.trigger("student_group");
 								}
 							});
+						}
 					},
 					function() {	//ifno
 						$(me.wrapper.find(".btn-mark-att")).attr("disabled", false);