I'll refer you to "The Straight Dope" to clear up your misconceptions about Bush's "acceptable" stint in the NG:
"By all accounts Bush was an excellent pilot, but apparently his enthusiasm cooled. In 1972, four years into his six-year guard commitment, he was asked to work for the campaign of Bush family friend Winton Blount, who was running for the U.S. Senate in Alabama. In May Bush requested a transfer to an Alabama Air National Guard unit with no planes and minimal duties. Bush's immediate superiors approved the transfer, but higher-ups said no. The matter was delayed for months. In August Bush missed his annual flight physical and was grounded. (Some have speculated that he was worried about failing a drug test--the Pentagon had instituted random screening in April.) In September he was ordered to report to a different unit of the Alabama guard, the 187th Tactical Reconnaissance Group in Montgomery. Bush says he did so, but his nominal superiors say they never saw the guy, there's no documentation he ever showed up, and not one of the six or seven hundred soldiers then in the unit has stepped forward to corroborate Bush's story.
After the November election Bush returned to Texas, but apparently didn't notify his old Texas guard unit for quite a while, if ever. The Boston Globe initially reported that he started putting in some serious duty time in May, June, and July of 1973 to make up for what he'd missed. But according to a piece in the New Republic, there's no evidence Bush did even that. Whatever the case, even though his superiors knew he'd blown off his duties, they never disciplined him. (No one's ever been shot at dawn for missing a weekend guard drill, but policy at the time was to put shirkers on active duty.) Indeed, when Bush decided to go to business school at Harvard in the fall of 1973, he requested and got an honorable discharge--eight months before his service was scheduled to end."
http://www.straightdope.com/columns/030411.html