Wow. That top picture of a cottonmouth is not like the ones I've seen (and I've seen plenty). Maybe that's a juvenile? The ones I've seen are in central and south Texas and they are invariably very dark in color, appearing almost purely black with only a few faint markings. And the heads are a much more pronounced triangular shape as I recall. And they are all fat-ish. Maybe 4 to 4-1/2 feet long and as big around as your forearm.

I guess I owe that science teacher an apology. But I would still hope that any teacher would make sure, and then make sure again, that they had correctly identified the snake. I would have pegged that upper picture you posted to be more similar to a copperhead than a cottonmouth. But again, I remember more distinct triangular heads on those too. I live in Colorado now and we don't have cottonmouths/copperheads here, so I haven't seen anything but a scattered rattlesnake for years and years.