sophomoric

/ˌsɑfoʊˈmɑrɪk/

Anything sophomoric is foolish and immature. It was totally sophomoric of your friends to throw our clothes into the swimming pool.

It's acceptable to use this adjective to mean "characteristic of or relating to a sophomore," so you could describe your sophomoric extracurriculars in high school, like marching band and chess club. It's far more common, however, for sophomoric to be derogatory, appropriate for describing your younger sister's ridiculous practical jokes or the class clown's antic. This word derives from contrasting Greek roots, sophos, "wise," and mōros, "foolish and dull."

Definitions of sophomoric

1. adjective

   displaying or suggesting a lack of maturity
   synonyms: adolescent, jejune, juvenile, puerile

   immature

   characteristic of a lack of maturity