"It is a long absence."
Translation:Es una ausencia larga.
¿Cual es la differencia entre "Es una ausencia larga" y "Es una larga ausencia"?
Adjectives in Spanish are normally after the noun.
However, when emphasis on the adjective is desired (For your example, it would be a really long absence), then it can come before the noun.
Here is a link to back it up: https://www.spanishdict.com/guide/adjective-placement
In this context of education it refers to missing class. Of someone being absent (not present) from class. Generally absent=not present
Why is it ser, being absent isnt permanent, unless in this case is it suggesting its permanent like a terminal ilness or they dont look like their coming back...
The "Temporary vs. Permanent" rule of thumb can be very problematic.
Instead, think of it more like "Character vs Condition".
In this case the "absence" is a characteristic of what "it" is
For an even more thorough explanation, see the link: