Dillon

Which is ironic since Anthony Rapp (who plays McGoo) has Jewish ancestry in real life.

I was taking a class and surprised to learn how the swastika was an ancient symbol with good meanings. I only knew the one.

It was either Dillon or that pencil necked geek who wore coke bottle glasses.

Before Hitler made the Swastika a symbol of hatred, vile, and everything else badit was actually a symbol of all that is good.
The Swastika Club in many high schools was similar to a sorority to complement Junior ROTC, especially in the '30s. It began to fade in the late 30s through today.
If you have a chance to look at a 30s high school yearbook, you'll see a lot of things that today are considered taboo/verboten.
And before you come at me with literary spittle, look it up.

It just happened to land there, that's all. I don't think David was aiming for McGoo there-he was just throwing it at the boys.

Is was either Magoo or Cole Hauser's character, although he reverses his views by the end of the movie.
To the world you may just be somebody, but to somebody you may just be the world.

LOL @ Chesty Smith

I am sure McGoo was among those who did it, but I don't think he was alone. Might be McGoo + his protector Chesty Smith. Or maybe even the whole group with McGoo as the creator of the idea.

I believe it was McGoo. When David Green throws the sign towards the group, it lands on the end of McGoo's hockey stick. He was also the one who made the most strident anti-semitic remarks throughout the entire film.
Its not hard to figure this one out.

I figure it was more than one; Dillon and McGoo, most likely. Dillon wouldn't want to do it alone, and McGoo kept sucking up to him.
Here's to the health of Cardinal Puff.

I think Dillon was the one. He had the most to lose from David and was extremely desperate.

It could as easily have been Chesty (the Ben Affleck character).

Probably Dillon. The idea is that we are left to guess who it was because most of these bigots are cowards and will do things like that anonymously. That is why the Klan wears hooded robes.