Answer:
I am a portal to another world which you cannot enter. Only you can see me but I can't see you.
Teenage girls are pros at creating me. Learn me in classes.
A young man wants to have me but when he has me he no longer wants me. Blade in hand he attacks me and does his best to remove me. Yet he knows that it is all in vain.
When you need me, you must find me. When you find me, you no longer need me. What am I?
It rows quickly with four oars but never comes out from under its own roof. What is it?
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.