Calling the Webservice class using Salesforce "SessionID" in Visualforce Page

Visualforce Page:

<apex:page id="page">
<apex:includeScript value="/soap/ajax/24.0/connection.js"/>
<apex:includeScript value="/soap/ajax/24.0/apex.js"/>
<script>
       sforce.connection.sessionId = '{!$Api.Session_ID}';
    //alert(sforce.connection.sessionId);
    function callapex(){
        sforce.apex.execute('scriptcls','m1',{s:'ctrla'});
                                }
</script>
  <apex:form id="form">
      <apex:pageBlock id="pb1" >
          <apex:outputLabel >Ctrl a</apex:outputLabel>
          <apex:commandButton value="Call an Apex Class" onclick="callapex()"/>
      </apex:pageBlock>
  </apex:form>
</apex:page>

Webservice Class:

global class scriptcls{
    webservice static void m1( string s ){
    System.debug('------------>'+s);
                                                          }      
                              }