We're looking for a word that contrasts with "some students may be innocent" and correctly agrees with the plural noun "students".

Much is used with uncountable nouns (e.g., much water, much money). “Students” is countable. A great deal is also used with uncountable nouns (e.g., a great deal of effort). A good deal, like “great deal,” is used with uncountable nouns.

But “most” is used with countable nouns and fits this context.