Which school of 1950s jazz emphasized heavy, dark timbres; featured tenor saxophone more often than alto; and used an assertive, hard-swinging drumming style?

Hard Bop

Explanation:

Hard Bop was birthed as an opposition to cool jazz of 1950s and was played mostly by urban musicians (from Philadelphia and Detroit) reflecting an East Coast, extroverted response to urban life.

While cool jazz preferred a light timbre, hard bop preferred a sound that was heavy, dark and impassioned. The saxophone of choice was tenor which replaced the alto and drummers played in a more assertive style.

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