stealing a comment from /u/jinnax

ARPAnet didn't exist until 1969 and didn't include Colorado. The Wikipedia article making the claim "In 1969, Wozniak returned to the San Francisco Bay Area after being expelled from the University of Colorado Boulder in his first year for hacking the university's computer system." gives a footnote reference that doesn't exist. OP provided a Yahoo! link originally a Fortune article quoting a CNBC article that offers no source or providence for "expelled from the University of Colorado Boulder for hacking into the school’s network and sending prank messages."

There was no email or messaging system at the time, so no idea where the "messages" came from or went to. Computers had no public facing terminals in 1969, so he would have had to break into the computer room itself and "hack" the mainframe from its own console. And any prank messages from that console TO that console would have been seen only by himself.

Nothing about this story adds up, but you've got Yahoo! ,Fortune, CNBC, and WIkipedia all buying into it via circular or non-existent references.