"They rationalize that by keeping it a secret they are protecting us (the LBS) but in reality they are protecting their own backsides against the backlash of their children, family and friends."
Exactly. Liars THINK they are doing it to protect YOU, when in reality they are doing it to protect THEMSELVES from the fallout of their actions.
My ex left when our youngest was 17. In retrospect, I think perhaps he had been thinking about it for 2 or 3 years, I was just oblivious. I'm sure he thought he was doing the "right thing" by "toughing it out" until the kids were almost out of the house.
I'm sorry you have to be reminded of OW. Is it a large campus where you would never have to see her? If so, try to think of it as just being in the same city as her. Focus on your fellow students. Make new friends. And if that doesn't work, think about transferring somewhere else next semester if you can??
(P.S. congrats on going back to school! My mom went to college when I was in grade school - back in the 60's at a time when it was very unusual for a 30-something mother of four to be going to college!)