"That boy is going to school."

Translation: वह लड़का स्कूल जा रहा है ।

[Christiaan627186]

Is 'jaa' a postposition in this sentence?

[vinay92]

No. जाना is the verb 'to go' and 'जा रहा है' is the masculine present continuous tense form 'is going'. The postposition between स्कूल and जा रहा है is implicit.