Student slang/observation. 置き勉 is a slang contraction meaning 'leaving one's study materials at school (instead of taking them home).' From 置く ('to place, to leave') + 勉強 ('study'). 没収される is the passive of 没収する ('to confiscate, to seize'). ~てる is casual for ~ている. よな is the reflective/confirmational final particle.