One of the reasons some people cannot see objects that are far away could be that they are short-sighted, they need see an eye doctor to know for sure. I suggest that you see an eye doctor to check your eyesight as soon as possible. They will give you have appropriate advice.
Without alarming you, you might be short-sighted that is being unable to see things that are a distance away. Is this something just specific to when you sit at the back of the classroom? Or do you experience this all the time when you want to see things that are far away?
Not to despair, you can correct this by prescription eye glasses. It is something an eye doctor, called an optician, can look at and advise.