Learn Polish: Szkoła jest po lewej stronie.

While po often means "after", it is also the standard word used to say "on" a specific side. You will always use po for expressions like "on the left" (po lewej stronie) and "on the right" (po prawej stronie).

What grammatical case are lewej and stronie in?

They are in the Locative case. The preposition po requires the Locative case when you are describing where something is physically located.

How did the dictionary word strona (side) change to stronie?

To form the Locative case for most feminine nouns ending in -a, the -a changes to -e. However, the consonant before it also softens. The "n" in strona changes to an "ni" sound, which is spelled with an "i" before the "e", giving us stronie.

Can I just say Szkoła jest po lewej and drop the word stronie?

Yes! Just like in English where you can say "on the left" instead of "on the left side", Polish speakers very often shorten this to just po lewej. Both versions are perfectly correct and natural.