What do you think about me being harsh to my students?
Why are you going to be harsh to them?
These kids ruined all of my plans.
They are trying to learn and you need to show more respect for the students in your class. They will do a lot better and learn more if you are  a little nicer.
They need to learn that cruel reality, if they want to get far, they will have to work hard
While they need to learn these life lessons, you being cruel towards to them isn't what will teach them. You need to be respectful when you're attempting to teach young adults anything or else they won't learn.