Green’s Dictionary of Slang

(US campus) to absent oneself from a recitation or class.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 23: blitz, v. i. To absent oneself from recitation.
1938		Des Moines Register (IA) 6 Mar. 14/4: When they do not go to classes they ‘blitz’ or ‘bleach’ them.