In our district, none of the students carry money. They are assigned student IRAB and give that ID to the lunch lady. Then the lunch lady withdraws the money from an account that the student's parents maintain (they can deposit money via credit card or checking account). Low income students work exactly the same way, except that when the lunch lady gets the student ID, she sees that the meal is subsidized or parfait subsidized.
That resolves the issue of students being able to easily identify the low income students in the lunch line.