The way I saw it was...just some random kid in the class. I think the creators of the show were making fun of themselves and purposely made mysterion a random kid that has similar features like everyone else in south park. Since the show is always criticized about how similar characters look especially their eyes.

Well it had to be somone, so i would guess...KENNY. coz he always has his hoodie up, right? And so did mysirion! and where was kenny at the crime scene? We saw kyle with him, so it isn't him, and we saw stan, and I'd say it was Kenny!! {I think Kenny's cute }!!

I think it was Clyde. Clyde was sleeping during Cartman's presentation (implying he had a late night) and was the only one to give an answer as to why he had Mysterion in his locker; an excuse?

It can't be Stan because he's always had a squeaky voice and never cared for any trends. Plus when he was at Kyle's he didn't reveal his identity, Stan would DEFINITELY had revealed his identity to his best friend.

So, it's either Clyde or just some generic kid.

ITS CLYDE

Its Clyde, he was asleep in class when Cartman was doing a presentation on 'The Coon' which implies he was out late the night before (Or he was incredibly bored)

He was also not crossed out on Butters board, and his voice is very similar to Mysterions.

Klyde. Wasn't Kyle or Stan, or Kenny(Kenny's voice is different a la the move and The Jeffersons) so it's Klyde. He hates Cartman the most out of background characters, so he is the likely candidate.(Or, food for thought, it could be Wendy. She REALLY hates cartman.)

People, don't choose after the voice. Cartman did his voice deeper as The Coon. So did the Mysterion kid.

But my best guess would be Craig. 'Cause he does the same things as Cartman, only better. And it just seems like Craig. Idk.
Clyde is a pussy who cries too easily. It doesn't fits with him being a superhero. But yet again, it doesn't fits with him being a pirat either. So, it could actually be him.

Butters also said that the body looked like Stan or Kyle's. But we very well know that it can't be Kyle.

Kyle?!?

Oh- whatever, you guys. Seriously, everyone knows joos can't be superheros.

its totally craig no doubt about it

hmmmmmmmmm it was not kyle because misterion was viting him when he was doing homework or somthing butt it could be that girl that says:who said misterion was a girl? oh right her name was wendy

Here's an easy explanation.
http://southpark.wikia.com/wiki/Mysterion

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!

Myserion was Stan.1.Myserion asks Kyle for help.2.Cartman naturally feels its Stan.3.Stan is a big part of the 4 boys and he is always trying to do something right.

It's Craig.Case closed.

It's Kenny
It's Defenatly Kenny

im pretty sure it waz kenny. cause he waz the last person anyone would suspect.

It is kenny working with stan, kenny always dies when he is in the episode,even if he dies in real life,(ex: in WOW) stan's uncle smuggles fireworks and kenny is too poor to afford them, so he must team with stan, and all of cartman's friends hate him. plus in one episode stan says to kyle that he is the smartest.