How come "szkoła" isn't in the instrumental case here? Is it because it isn't noun = noun?
Ten budynek jest szkołą = This building is a school.
After "to jest" you use the nomative form.
Is there a reason nominative has to be used or is “to jest …” an exeption and I have to memorize it?
Why an exception? In a simple "This/That/It is Y" sentence, Y is always in Nominative.
An exception, because usually only the subject of a sentence is in nominative and here the subject is “to”, isn't it?
Well... yeah, I guess it is. There are not many situations when you use Nominative, but for sure it's here, obviously the subject and a construction "jako X" (as X, for example 'working as X').
This is an answer to Jellei, there the „Reply“ link disappeared…
Thanks, I think I get it now. And I think it should have occured to me way earlier…
Can you say 'jest szkolą', '[it] is a school', in the same way you might say 'jestem kobietą', '[I] am a woman'?
Not when the subject of the sentence is a dummy pronoun, i.e. "[It/That/This] is". Or any other pronoun, actually. So no, not here.
Possibly "Ten budynek jest szkołą" (This building is a school), although it sounds a bit strange to me.
is there a way of writing this so that we don't need the English article a in the translation?
I don't think so. No articles in Polish, and it is only grammatically correct to write the English with them.