<%
StudentID=Request("StudentID")
ID=Request("ID")
Sname=Request("Sname")
Subject=Request("Subject")
if StudentID="" then %>
<script language=vbscript>  
   MsgBox "错误：请输入你的准考证号！"  
   location.href = "javascript:history.back()"  
  </script> 
  <%
  end if
......................