Anecdotes like yours don't prove anything. That's why we have science.
Without a control group and test group we don't know if your conclusions are statistically valid. Maybe most people get better grades when they watch TV and play video games.
I probably mis-worded my thoughts. I did not meant "it's true for x therefore it's true for the whole set". It's definetly not something we can generalize. At least, we can say that he's not completely wrong.
Nevertheless, I still believe that if such study was conducted, it would tend in that direction. The actual difference between a multi-tasker and a uni-tasker might not be very large, there might even not be any difference in some cases. Considering how the memory supposedly works, I find it hard to believe that one would retain the equivalent amount of information while multi-tasking than while uni-tasking.
Without a control group and test group we don't know if your conclusions are statistically valid. Maybe most people get better grades when they watch TV and play video games.