I have student identification from a school outside of Norway, is it valid as student ID?

No.

To be entitled to the student discount, you need student identification from a school in Norway.

For students from schools outside of Norway, the ISIC card is the only valid identification.

ISIC (International Student Identity Card) is the only internationally approved student ID and it is sold and used worldwide. Read more about ISIC in Norway or order your card at ISIC.no or ISIC Norway on Facebook.