This is the 705th most frequent Icelandic word.
skólann
"The school" (definite form of "school").
The word "skólann" is in the accusative case, which is used for the direct object of the action, indicating that the walking activity involved movement inside the school.
Við gengum um skólann og skoðuðum bekkina.
We walked around the school and looked at the classrooms.
The word "skólann" is in the accusative case here as well, indicating a relationship to the purpose (for) of the books.
Bækurnar fyrir skólann voru mjög dýrar.
The books for the school were very expensive.
The word "skólans" here is in the possessive form, indicating that something belongs to the school or is associated with it.