What is the hidden achievement that I'm missing for LEGO Harry Potter Years 1-4 iOS?

Solution 1:

There looks to be two hidden achievements out of 34 total. You've already completed one as 33/34 achievements are listed above, so your remaining challenge will be either:

  • "Quiet, please", which solution can be found here
  • Avoiding the prefects on year 3, as in holly's very similar topic here

In both cases, all achievements were listed and described on supercheat