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/HomepageTop.php?teacher_id=-99'+union+select+0,1,teacher_password,teacher_account,4,5+from+teacher/*