I think it is just implied when you mention a destination with the verb "जाना".

Ko is used for an object. When some work is being done on an object, ko is used.

में means "in". मे by itself is the sound "me", but I don't know that it is a word. A native speaker might have to answer that.

If you said "स्कुल में", you would be talking about actually being in the school, rather than going to it. There is an implied "को" postposition in this sentence.