Series of novellas about a video game and weird silly putty

I’m trying to identify a series of short novels I started reading in high school.
I read the books at some point between 2007 and 2010 but the books seemed older based on the condition they were in, maybe late 90s/early 2000s.
The story was about a boy who discovered some weird video game and shared it with his English teacher from school. The players got to create their own characters out of clay golems and design them to their own liking with the English teacher opting to make his character a "blonde bombshell".
I also seem to recall that there was some sort of silly putty that seemed to relate to the video game somehow.
There were rumours going around that kids with the putty were getting sick and disappearing. A very specific detail I remember was that there was a competition to win a Renault Twingo (or Clio?) car if you collected enough silly putty lids.
The books themselves were only around 150-200 pages each, printed in smaller form factor books. Seemed like there was a series of maybe 5-6 of them, but I only read the first 2 or so.