So who gave her the idea she can't do math?

I substitute taught the past two days--one day with learning and emotionally disabled 7/8 th graders. They were a group of only eight and were just terrific kids. Very broad range of difficulties from mild retardation to pretty severe emotional issues. One boy seemed totally fine--smart, helpful, well-mannered. If I were his parents I'd be wondering when he was going to be in the mainstream classes...and then there was my first grade class today. Thirty-one 6 year olds. Our school district is getting terribly overcrowded (the teachers are poised to go on strike any day now) and there is just no way that quality education can be maintained with that many young children in one class with one teacher. During the Math lesson I was trying to teach them the different ways to add numbers to equal "7" and then "8." Several of them just couldn't get it and they should have been pulled out and given one-on-one help but you can't do that. The kicker was when a little girl, after repeatedly expressing the opinion that she could not do the work finally blurted, "I'm Mexican. I can't do math." OMG. Where on earth did that come from? It took me several seconds to muster an answer but when I did it was to the effect that it didn't matter if a child is Caucasian, Mexican (which I realize "officially" is still considered a type of Caucasian), or Korean. All children can learn Math. She didn't have any accent whatsoever and I've taught the bilingual first grade at that school and if she truly had language issues she'd be in it. But she expressed herself very clearly. So who gave her the idea she can't do math?
Anyhoo, I got so involved in the topic that I didn't notice we were out of time and the end-of-day bell rang and mass panic ensued. Little bodies were heaped upon each other as they jostled for coats and backpacks and such. I kept saying Everyone, calm down, it's okay, but I may as well have been barking at the moon. At least no one missed their bus and only one lunch box was forgotten.