That’s interesting. In my experience the A-Level themes are made to be very general/conceptual so that students can interpret them in whichever way they like without feeling creatively restricted - so essentially you can link anything you want back to the theme if you like! For example, think about themes/ideas/images/styles that you want to create, and then think how you could link that back to the ‘conceal and reveal’ (working backwards, rather than ‘conceal and reveal’ as your starting point). Then you can do what you actually want to do rather than getting caught up over an arbitrary theme.

If you do want to start with the theme though, ‘conceal and reveal’ makes me think of social presentation vs inner reality, the various ways people hide themselves or change who/what they really are - e.g. Facebook and social media, make-up, white lies, photoshopping in the fashion industry etc.
Or you could link it larger-scale issues - governments, media etc.

Think about areas of life and reality in which things are distorted, people lie, things aren’t what they seem. You could link it to illusions - magicians, the occult, the paranormal. 
I could probably keep going! The main thing is don’t allow the theme to trip you up or restrict your thinking - maybe try just writing words down that come into your head, see where they lead and so on and so on. Essentially there’s no wrong answer!

Hope that helps!