Here's my take:
2 versions of each. (second ones are lazy edits)
Lets call them:
"Prep," "Jock"
"Nerd," "Loner"
The first major change is that they are posed differently from each other, and have different postures.
"Prep" has the best posture, the kind that comes from confidence or cockyness, and he probably has both. "Jock" is more hunched over, and bigger. More threatening. Like a lineman. He knows he can do some damage. (His legs should be spread more apart and his butt should be a little more flung back but I got lazy, ok?)
"Nerd"s posture is a little slumped at his shoulders. He has a sort of awkward stance, back slightly hunched and pelvis a little forward. He is not in a fighting stance because he's a thinker, not a fighter. (That doesnt mean he won't fight if it comes down to it.) If you want more of a fighting stance, I'd recommend putting his hands closer to his face than the "Jock"s. Kind of in a protective way. "Loner" has a similar posture (and in fact I simply flipped "Nerd"s body around. He has turned his back on the world. (Because the world turned its back on him?) It is honestly an extremely silly stance to think someone would stand in, let alone fight in, but its pretty common in RPGs and lets be serious, it looks pretty cool.
The next thing is their shirts. I was reading this post and people suggested putting a brand on the popular kids shirt, but in my experience, popular kids wear clothes either with
- vague "beachy" graphics
faded something or other
Blahblahblah EST. 19XX
shirts with little insignias
of those, the insignia is by far the easiest to represent in pixels.
I did decide, however, to give the nerd a shirt with a logo. Because we love our bands/videogames/tech companies/comics. Nerds pride themselves on their interests. Jock/Preps pride themselves on how much they can bench. (This is of course, a joke, and extremely oversimplified, but the point is valid)
Because it has to fit on a small amount of pixels, its just some vague boxy thing. Probably a tech company?
I kept the "Nerd"s hair how it was, but gave the "Prep" a haircut.
"Prep" has kind of sort of (hard to pixel) faded jeans, "Nerd" has some darker plainer (reasonably priced...) jeans.
"Nerd" has some wristbands. (Maybe they have skulls on them?

)"Prep" could definitely use a belt. Ran out of colors. Whatever.
"Prep" has a darker skin tone from spending more time on the beach slammin brewskis with his brodogs... or whatever.
"Nerd" has a lighter skin tone from spending more time pwning n00bs at Little Big World of Minecraft with his l33t h4xxor skills... or whatever.
...
...
(oh man. how amazing would that game be?)