Subject: What if two Lewis structures for a molecule have the same formal charge?

In my OAC-AP (Ontario Academic Credit, Advanced Placement) level chemistry 
course, we learned that many molecules and ions have more than one Lewis 
structure, and that we can find the most acceptable model by finding the 
formal charge of the ions/molecules. We used the model of a sulphate ion 
to see that the model with two single bonded oxygens and two double bonded 
oxygens around a central sulphur atom was the most acceptable. However, 
when we were drawing possible models for the ion, one of my classmates 
drew the ion as a chain (O-O-S-O-O), and it technically followed the rules 
for Lewis structures as we have learned them so far. It also has the same 
formal charge as the proper sulphate ion. Our teacher said that the 
sulphur ion with the two single and two double bonds is the only one they 
have found to be possible experimentally, but that he doesn't know why the 
chain structure doesn't work/isn't possible. Why is it not possible?

Re: What if two Lewis structures for a molecule have the same formal charge?