Actually you can get a lot of gain, but not 100%. This is not really the
worst pretest you can get.
If you give the Lawson test from his book, and multiply the %score by 1.2
that is a limit for each student normalized gain on the FMCE. So I assume
it would be similar for the FCI.
When students score below random chance, that is a sign that at least they
are thinking because they are selecting the common misconceptions rather
than drawing christmas trees.
13/30 is higher than I have often seen on the FMCE, so they have room to
grow. If they all came in with 100% you would have little to do other than
give them all A and pass them on. If it were 15/30 you have a shot at 100%
gain.
As to ability, that is a matter of what thinking skills they have acquired,
and the Lawson test is a short overall test of this. If they all have
proportional reasoning, you are very fortunate. This might even be a class
with 75% having proportional reasoning.