I'm just playing devil's advocate here.
Could he not go home for a weekend while at grad school to perform Guard duties? Or schedule two weeks away to fulfill his other commitment to Guard duty? Or be temporarily assigned to another Guard unit?
Further, "Just out of grad school" isn't very specific. Was he out for days, weeks, months, years? Did he go to grad school full time? What?
It seems no matter what is brought forward, those who want to believe Bush was AWOL will discredit the source.
I just read in a New Jersey paper yesterday an article which quotes Joe Lefevers, a member of the 187th in 1972 who said he remembers seeing Bush in unit offices and being told that Bush was in Montgomery to work on Blount's campaign. "I was going in the orderly room over there one day, and they said, "This is Lt. Bush," Lefevers said yesterday. "they pointed him out to me...The reason I remember it is because I associate him with Red Blount."
I couldn't find a link to this article but I still have the article here in front of me. It was in the Star Ledger and the story comes from the Birmingham News and it was written by Mary Orndorff and Brett J. Blackledge.