You must be polite to your superiors.

Literal

Superiors [to-には] politely must-do.

目上の人 ('people above one's eyes' = superiors in age or rank) is the standard term. 礼儀正しくする ('behave politely') uses the adverbial form of the na-adjective 礼儀正しい. ~なければならない is the formal 'must.'