Alternative form of test the waters

While flying; in flight. / (figuratively) In motion. / (figuratively) Departing.

Alternative form of play both sides against the middle

(colloquial, only with a negative) Even if one's (own) life depended on it; at all.

(Britain, humorous) Intentional misrendering of the proverb "the world is one's oyster".

plural of man in the street

(idiomatic) A typical person, unversed in a given subject; an inexpert commoner.

To substantially control a school administratively, financially, or in terms of its curriculum.

(slang) To die. / (slang) To break down; to be impossible to repair or not worth repairing. / (idiomatic) To perform poorly; to fail. / (slang) To be unpleasant or undesirable.

(idiomatic, chiefly UK) Alternative form of grist for the mill