Ok, so let me try to understand this from your somewhat sketchy description -

Your teacher wants you and your classmates to show him how to print receipts?

Ok, so show him - do it in class where the situation is monitored by the rest of your class, and passing teachers/administration. Yes, be sure to mention it to your parents and other teachers because caution is always wise. If there's not a computer in your classroom tell your school's principal to get with the 21st century and make the teacher bring his laptop if he wants to be shown.