How to remove sulfuric acid from sample
Sulfuric acid is an excellent dessicant and is also very non-volatile itself (it boils at 337&nbsp;&deg;C).  In other words it would likely still have been there in yet another month.

Your instructor was correct to suggest that you rinse the sulfuric acid with some cold water.  Benzoic acid does have some non-negligible solubility in water (1.7&nbsp;g/L at 0&nbsp;&deg;C).  Unfortunately, it is even more soluble in most organic solvents so cold water may well be your best bet.  The solubility in 25&nbsp;&deg;C water is 3.4&nbsp;g/L, twice the solubility that it has at 0&nbsp;&deg;C, so definitely keep it cold. 

Additionally, use as little water as necessary to get the sulfuric acid out.  If you are working with a gram or so of benzoic acid, I would start with a dropwise addition of about 10&nbsp;mL. If it’s still gooey, or still appears wet the next day, you need to repeat the rinse. It’s best if you can use a vaccum filter system of course, and spread the material thin and even before rinsing.