Why some things we learn about in school won’t help us in real life.
Why is this true? Why do teachers do this?
That depends upon how you interpret "real life." In my experience kids don't know what they want to do -- and some don't realize it until after they are finished with college.
Many years ago I taught a six-week course to a small group of 7th grade boys. They had the same complaint you do. I bet them $100 that they would not grow up to do the job they thought they would. I could have paid it to the boy, son of missionaries, who said correctly that he would be a missionary. He's the only one. The boy who planned to be a police officer was in jail on illegal drug charges. None of the others fulfilled their goals either.
I understand what you are saying but lets say for an example that this happened,
This year when I was in my 6th hour and we had a packet of papers that we had to get done before the end of the week. We came in the following day and we pulled our packets out and started to work on them because we knew that we had limited time until they were due. After the bell rang my teacher stood up and told us to pull out our packets after the few that hadn't pulled them out, got them. I expected my teacher to tell us to get straight to work until the bell rang for us to go home, instead my teacher told us to rip up those packets and throw them away. We all couldn't believe our ears, had our teacher really told us to rip up our papers and throw them away. After we got done throwing them away my teacher than assigned us a new assignment. We later found out that he didn't agree with what our district told him to do.
What would you say for this instance?