This is the 3781st most frequent Russian word.
школьницу
"Schoolgirl" (accusative case).
In this sentence, 'школьницу' is in the accusative case, indicating the direct object of the sentence.
Я увидел школьницу на автобусной остановке.
I saw a schoolgirl at the bus stop.
Here, 'школьницу' is also in the accusative case, functioning as the object of the verb 'похвалил'.
Учитель похвалил школьницу за её старания.
The teacher praised the schoolgirl for her efforts.
In this sentence, 'школьнице' is in the dative case, indicating the indirect object of the verb 'дали'.