Here's an easy explanation.
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But here's my explanation. Let's take a look at everyone in the class, shall we?
Cartman, Kyle, Stan, Kenny, Butters, Tweek, Craig, Clyde, Jimmy, Timmy, Token, Kevin, and Wendy.
Obviously Mysterion isn't Cartman or Butters because Cartman is the Coon and Butters is Professor Chaos.
This leaves Kyle, Stan, Kenny, Tweek, Craig, Clyde, Jimmy, Timmy, Token, Kevin, and Wendy.
Mysterion can't be Jimmy because Jimmy stutters when speaking and wears crutches, Timmy because Timmy can only say his name and his head is large, and Token because Mysterion has revealed himself to be white.
This leaves Stan, Kenny, Craig, Clyde, Tweek, Kevin, and Wendy.
When Mysterion unmasks themself, Cartman proclaims that it is someone he had previously mentioned. This leaves Stan, Kenny, Craig, Clyde, and Wendy.
When fireworks and voice are taken into consideration, Kenny couldn't afford fireworks and Wendy's voice isn't that deep.
This leaves Stan, Craig, and Clyde.
Craig may be Mysterion because his voice deeply resembles Mysterion's.
Stan may be Mysterion because his uncle Jimbo could have obtained the fireworks for him.
And Clyde was asleep in class, proving that he was up all night crime-fighting, possibly?
(Plus, Mysterion's voice sounds like all of them.)
But it's not really known yet!