Buick - Century :: 1991 Won't Start - Only Got A Slow Crank Like A Low Battery

I bought a well-used 1991 Buick Century for my son in December '12. At that point I put in a battery that was bought brand new just the month before. The car ran fine until a couple weeks ago. He tried to start it but only got a slow crank, like a low battery. I tried to jump start it, with no difference. Since the car was at his school, I asked his auto shop teacher to take a look at it. The teacher tested the battery on two different machines and determined the battery was bad. I returned it to the store I bought it at, where it was tested as working fine. I then swapped the battery from my car for his, and his battery (the one that was tested bad) started my car with no problems while my battery would not start his car. It produced the same slow crank. I talked to the shop teacher again, who pulled the starter and took both the starter and the battery to various auto parts stores. The starter tested good, but the battery was tested bad at 3 shops, and weak at a fourth. So the car is still sitting at the school, and we're at a loss. What might be causing this problem and how can we get his car running again, short of just buying another car?