Isn't that a greek letter?

Yes - it's actually three greek letters. Sigma Alpha Epsilon.

I see you are from Romania. Let me try to explain.

At our colleges in America, we have groups of wealthy and needy boys who decide to call themselves "brothers" because they each pay "fees" each month into a giant fund that gets spent on liquor, party supplies, houses, hookers, and date rape drugs. They are each represented by three greek characters and call themselves "Fraternities" (like because they apparently become fraternal by sleeping in eachother's puke). The main goal of the whole thing is to act like you're a little cooler than any of the other fraternities, and way cooler than anyone not in a fraternity, so by all paying to be best friends with each other you'll always be the coolest people around. As such, they frequently wear around matching t-shirts with three greek letters on them. The fraternities are then represented by "National Chapters;" so for example, there is a "Phi Delta Theta" national board, and then there is a "Phi Delta Theta" at a whole **** ton of Universities across America.

So someone from this "chapter" of the fraternity "SAE" (Sigma Alpha Epsilon) at the University of Oklahoma (or someone on the bus with them) decided it would be a pretty stellar idea to take a video of everyone singing a chant about how black people aren't allowed in SAE and should instead be hanged from trees and release it all over Youtube. The US had been in like an uproar about the video originally, but even more-so now because the kids got expelled from college, and they say "it's freedom of speech and happened off campus, how can you expel someone for that?" But then other people are furious nonetheless, and they've been investigating SAE chapters all over the country ever since. It's been all over the news here though.

I've never seen American Pie, but those actors are hilarious. I would guess if it's about college it probably has some fraternity references.

Yes those are the shirts - TKE (they say it like "teek") is a fraternity at a lot of colleges.

I'm not a huge fan of fraternities, but they can do a lot of good for people in a lot of occasions. Some take their philanthropic efforts pretty seriously, and a small handful aren't all about partying and saying "bro" a lot. I think most Universities only let the others continue to operate because they're a huge draw for incoming Freshman students.