it is absolutely %100 alvin.
if you take a look at the original picture, you'll notice that brittany is really the only one using technique, while the others are not. this is already in alvins favor.
next, you notice that alvin is doing the jordon, this is clearly a sign of good head, his face appears close to climax, while theodore looks like he's about to cry and simon is having to take control of jeanette, clearly a sign of bad head.
let's also not forget that alvin and brittany are both super popular
(alvin's a jock, as he was playing football in the squeakquel
, while brittany is a bit of a bad bitch, so even if she isn't liked, she probably gave head to make friends.
), so it is safe to assume brittany has given before, and alvin has received before, allowing brittany to give alvin better head with previous technique and experience. Theodore is fat and simon is unpopular, so this is most definitely their first time, so it's safe to say it's awkward.