When testing the code:

<event type="process-start" >
<script>
System.out.println("starting process");
</script>
</event>

A validation error is thrown that <script> is not allowed here, however, it is according to the jbpm docs: http://docs.jboss.com/jbpm/v3.2/userguide/html/processmodelling.html#script

When removing the schema, it works fine, so looks to be an issue with the xsd.