The abstruse style of the speaker left the students disinterested and fidgety.

'Abstruse' means difficult to understand, obscure, or hard to comprehend. 'Recondite' is its closest synonym, meaning dealing with profound, obscure, or abstruse subject matter. 'Erudite' refers to being learned/scholarly, which is different. 'Academic' and 'concise' do not match the meaning.