source: http://www.securityfocus.com/bid/29372/info
  
ClassSystem is prone to multiple input-validation vulnerabilities because it fails to sufficiently sanitize user-supplied data. These issues include multiple SQL-injection vulnerabilities and an arbitrary-file-upload vulnerability.
  
Exploiting these issues could allow an attacker to compromise the application, execute arbitrary code, access or modify data, or exploit latent vulnerabilities in the underlying database.
  
ClassSystem 2 and 2.3 are affected; other versions may also be vulnerable. 

http://www.example.com/class/MessageReply.php?teacher_id=1&message_id=-99'+union+select+teacher_account,teacher_password,3,4+from+teacher/*