How do I politely ask a convenor to not publish my name on the conference website? I will be presenting a paper at a major conference in the Spring: this will be my academic debut! 

The issue is that I have a permament teaching job (middle school) that does not permit time off to pursue academic pursuits: how ironic.  This conference entails an inter-continental trip and hence, three "sick days" off. Cough, cough. 

My name is already up on the conference website for anyone to Google, i.e. my employer.

How do I politely ask the convenor of my session to hide my name or alter it in some form? Withrawal is not an option.
From both an ethical and a practical standpoint, you don't. The ethical reason should be obvious (and has been stated in comments); you are lying to your employer about leave, and asking the conference organisers to be complicit in this (presumably without their knowledge). If you want to take sick time when it's not permitted, that is your risk to take, don't ask others to shoulder that for you.

From a practical standpoint, you should really ask yourself if hiding your name in the conference proceedings is in your best interests (relative to the chances of your employer discovering via Google what you were up to). I will presume you are going to this conference to begin/advance/change to an academic career. Reputation is gold in academia, and by concealing your activity, you are actively hurting yourself and reducing the profile of your work. If you don't care about the profile of your work and your academic footprint, then why go to a conference? They are expensive, and there are much nicer things to do with your time on a transcontinental trip.