Is it a preposition, or a conjunction?

Hi teachers!

I wonder what word class the underlined "rather than" is in this sentence. I think it can either be a preposition or a conjunction, and "Rather than being engaged" is also grammatical.

Please let me know whether I am right or not.

" Rather than be engaged actively in the lesson, he may have been preoccupied with trying to imagine pumpkin pie: What does it taste like? How does it smell? Is its texture chunky like apple or cherry pie?"

Than you very much.