So why "Miss"? I think it's probably simply easier to say. If you'll notice, very few people actually say "Mrs.," even if it's with the last name--we've taken the 2 syllable word and turned it into "Ms." This may or may not be connected to the Women's Lib movement of the 60s.
Your non-Hispanic students have simply picked up the habit from their friends. Often, if it bothers a teacher, students will continue it just to "push a button."
Interestingly, by the end of my public school career, the tendency for students to use "Miss" had lessened considerably, this in a district that is about 85% Hispanic.
It's a complicated issue that generally just requires a bit of explanation to someone who doesn't understand.