Learn Russian: Они создали новую школу.

Because the school is the direct object of the verb создали (created), it must be in the accusative case. The feminine nominative phrase новая школа changes its endings to -ую for the adjective and -у for the noun, becoming новую школу.

What is the infinitive of создали and is it perfective or imperfective?

The infinitive is создать (to create) and it is perfective. Using the perfective aspect here shows that the creation of the school was a single, successfully completed event in the past.

Why does the verb создали end in -ли?

In the Russian past tense, verbs do not conjugate by person (I, you, he), but they must agree with the subject's gender and number. Because the subject is the plural pronoun Они (they), the verb takes the plural past tense ending -ли.

Since this lesson is about the instrumental plural, how could we add an instrumental plural phrase to this sentence?

This sentence currently uses the accusative case for its object. However, you could add the instrument they used to build it! For example, Они создали новую школу своими руками (They created a new school with their own hands). The endings -ими and -ами show the instrumental plural.