The problem in the case options you have defined is probably that it will be matching a regular (textual) expression, not a numeric range. Consider the characters that your variable is range than the value.
Does that help? A good case statement can make a wonderful replacement to if...then...elif...elif...elif...fi or lots of separate and confusing if statements. Just as good programming technique, your should follow this though and I would expect you would to get a better mark if you can crack it.