Copied from a previous comment of mine:

Danish utilises V2 word order which means that the finite verb has to come in the second position in a declarative main clause (= a main clause that isn't a question).

You see that in the following sentence, drikker comes in the second position: Jeg (1) drikker (2) kaffe med mælk (6).

If we add something in front of the sentence, the verb still has to be in the second position:

• Generelt (1) drikker (2) jeg (3) kaffe med mælk (6).

• Hver anden torsdag (1) drikker (2) jeg (3) kaffe med mælk (6).

"I skole" is general, like "Børn skal gå i skole hver dag", or "I skole lærer børn om historie" (in every school children learn about history).

"I skolen" is specific. In this sentence, it really means more like "In this particular school, they are looking at the computer screens all day". Other examples: "I skolen er lærerne gode" (in this school the teachers are good) , "Børnene glæder sig i skolen" (the children look forward to going to this school).