they only do A-Levels to do the 'norm' because someone of their class and status cant not do A-Levels it would look bad.
Then again you are correct in that none of the royal family are actually bright. This is most likely due to everything being done for them, so they have no real motivation for self-improvement etc.
As long as they look good and speak well, and continue the royal line thats all that needs to be done.
I Think charles and william didnt do well either, you are correct all the public surrouding and lime light generally doesnt make well for study.