Learn Hardcore Norwegian: Skolen arrangerer en konsert i morgen. - The school is organizing a concert tomorrow.

En is the indefinite article for singular common-gender nouns in Bokmål. Putting en before konsert makes it “a concert” rather than “the concert.”

What gender is konsert, and how do I know to use en?

In Bokmål, konsert is a common-gender noun (historically masculine). Common-gender nouns take en as the indefinite article. Although some dialects or Nynorsk may use ei konsert, standard Bokmål uses en.

What tense is arrangerer, and why is the present tense used if the concert is tomorrow?

Arrangerer is the present tense of arrangere (“to arrange/organize”). In Norwegian the present tense often covers scheduled future events—much like “The train leaves at six” in English. If you want to emphasize future you can also say Skolen skal arrangere en konsert i morgen (“The school will arrange a concert tomorrow”).

Why is i morgen placed at the end of the sentence, and can I move it elsewhere?

i morgen (“tomorrow”) is a time adverbial. With the subject at the start, time phrases normally go after the object:
Skolen arrangerer en konsert i morgen.
You can also front it, but Norwegian obeys the V2 (verb‐second) rule, so the verb still comes right after your time phrase:
I morgen arrangerer skolen en konsert.

Does arrangere require a preposition before its object?

No. Arrangere is a transitive verb that takes a direct object without any preposition. You simply say arrangere noe, for example arrangere en konsert.

Is i morgen always two words in Norwegian?

Yes. In both Bokmål and Nynorsk it’s two words: i morgen. Writing imorgen is non-standard.