Sonic the Hedgehog.
Which I didn't play until '99. Sounds odd I know, but hear me out. Back when I was first starting school Sonic was everywhere. TV shows, comics, toys, books, food, bedspreads, you name it, you could buy it with Sonic's blue grinning face on it. Hence practically every kid at school was a major Sonic fan, it was the in trend, not just in gaming, but the hottest fad. If you didn't have Sonic stuff, nobody wanted to know. But it was a bond of Sonic that brought me some of my closest friends. Sure, as time went on, we weren't as hyped as were about Sonic, but I remember having arguments over whether Tails was better than Knuckles, and playing pretend we were Sonic in the playground, writing our school work with Sonic pencils and generally just being good friends who initially bonded over a blue hedgehog.
Yes, there were other games I got into, and other games I played a shit tonne as a kid (such as Crash Bandicoot for one example), but Sonic's the thing I think of when I remember being a kid starting school.