"Moja siostra jest w szkole."

Translation: My sister is at school.

[sharinglanguage]

I am assuming "szkole" is the locative case of "szkoła".

[Jellei]

And assuming correctly.

[Nngpi]

Does this mean 'my sister is physically in the school building' or something like 'my sister goes to school' (or both)?

[Exonym]

Until a native speaker chimes in with a better (correct) answer: I think it means both "My sister is (physically) in (the) school (building)" and "My sister is (enrolled) in (and attends) school". However, to say my sister 'goes' (in a similar meaning to the latter sentence) to school is probably better phrased as "Moja siostra chodzi do szkoły".