"High Risk Website Blocked" - Mal/HTMLGen-A has been found on this website

I am a website developer and one of my websites is being tagged as a high risk website by Sophos Anti-Virus.  The URL is www.myschoolerp.org. I did a test on www.virustotal.com and Sophos is returing a "Malicious site" result. This is a very simple website and absolutely nothing malicious has been installed on it.  My web host company has tested the server at their end and cannot find any problems. I tested the website on Google Webmaster tools and nothing negative resulted. I tested the site on https://trustedsource.org/and the result was 100% positive.  I install sophos home edition on my server and nothing has been detected. Can you please advise why your software is having a problem with this website and what code is resulting in the website being seen as a problem? Thanks so much for your assistance.