Tuning out one thing is simple enough. You just have to let yourself get used to it. Tuning out the world however is harder than that. First though you should know what "tuning out the world" means. It means that you can stop yourself from hearing what you don't want to hear. Allowing yourself to focus on what you want to focus on.
There are going to be places that naturally induce this and to varying degrees. Of course these places will vary somewhat from person to person. The places that do this for me are school(of course), church, and the bus. School has the effect of giving me day dreams. Church causes me to basically pass out where I sit. On the bus it is like being lulled to a kind of trance where you decide what you watch.
Day dreams are one of the easiest and most common type of this tuning. They are induced by boredom and you thinking further into a topic in your mind. This topic can be anything from fantasies to the last test you had, but it has to intrest you or inducing a day dream will fail.
Sleep is without a doubt the easiest and most common type of this tuning. However it is one of, if not the hardest, to control. I will not teach you to control your sleep. Inducing sleep is another matter (there are of course no garuntees to any of these methods working for you). You have to hind a comfy position and, unlike daydreams, focus on being tired. Soon enough you will start to yawn. If you don't get disturbed you will fall asleep. This should only be done when you have a couple hours to kill.
This is one kind of tuning that requires the most time to master. It is one of the most advanced kinds of tuning out there. This tuning if mastered can give you the benefits of multiple other kinds of tuning. It will give you dreams like a day dream. You will feel more rested when you exit this half-state, but not as rested as if you had slept. It will allow you to pass less time than sleeping, but more than day dreaming. If done correctly and with experience you will be able to exit the half-state when you are being talked to or about near you. Inducing this is difficult. You have to not tune out the world, letting in everything around you in to your mind. While doing that you must then focus on one topic while still letting everything in. Then seperate the 2. Allow yourself to maintain a connection to both the dream and the world (in other words RELAX). This will split your conciousness into 3. One part is sleeping. Another paying attention to the world. The last part is where you are focused the most allowing you to ignore the others until necesary. The last part is watching and listening to the day dream.
I do not endorse doing any of these in school. If it fails, you either aren't paying enough attention to the right thing or you are paying too much attention to anything.
If you read that all then congratz.
If it fails try again.
If it works for you then awesome.