I believe there absolutely was an intent to poke and prod the Mexican kids. The thing is, we don't know whether the Mexican kids had been being "in your face" and douchey about things prior to that. We don't know who initially started using the day as a way to rub the other group's face in something. Using their own national pride as a provocation.
It could have been the Mexican kids, or the white kids. I don't believe any of us here know that definitively. I know who I strongly suspect was the first to be douchey, based on other CA school stories similar to this I've read. But I could be wrong.
Even if the white kids were first, though, I would have some sympathy with them trying to stand up for their heritage and their nation if they were feeling like their area was just being completely swamped and taken over. Which is a common, and an accurate feeling to have in parts of CA in particular.
In 1950, California was about 94% white. By 2011 that percentage was down to 40%. Could you see how that might make a people feel like they were being pushed out, displaced, ethnically cleansed from an area? How it might make them feel culturally threatened and that they might, just maybe, want to push back against that ever so gently by expressing their pride in their own history? I don't find that racist or hateful - I find that completely natural.
Ultimately I don't care what their intention was, it's the principle of the thing from my point of view. I absolutely support the right of both groups to do what they did, on any day of the year. I particularly support the "USA!" kids because of what country this took place in. I disagree with the Mexican kids being here, but as long as they are here I would never seek to deny them such a fundamental right.
Source: Washington Post and Harvard
You'll note that the Scandinavian countries have the darkest orange, or least diverse ranking possible (though you're certainly right if you're wondering how long this will last.) Also worth noting is the fact that Canada is shown as being very diverse, but would not be considered so by most people using modern definitions of diversity because it is gaining that ranking primarily through having the full rainbow of different types of European heritage.
I've put in a map of world happiness ratings just for a fun additional thing to compare with.