Help with manners

I am an aide in a middle school class. I'm very dedicated and have years of experience but still struggle with responding to students (letting them push me when I should be tougher.) I handle the materials so kids will say to me "I ain't got nothing to write with, I ain't got a pencil." I say can you ask nicely? And they do then but go back to the rude behavior next time. How can I stop this so they ask politely every time or at least most times?