Ever wondered why your mind keeps useless information and not retain important stuff?

Thinking back to my high school days recently, the conclusion came to me, that I hardly remember any of it. I remember I had a biology class I really liked, but I can’t remember who taught it or what I learned in it. I also had an ancient world history class I liked a lot, but hardly remember anything of it. The only teacher I can remember is my art teacher. Other names come to mind but I don’t know if they actually taught at my high school, if they were from Jr. high school, or if I ever had a class they taught. However, the theme song to ”The Love Boat”, ”Gilligan’s Island”, and other useless information I remember even though I didn’t try to. If this has happened to you, have you ever wondered why? Why can’t the stuff that is important be remembered as well as worthless junk?