Aura Trample removed by High Priest of Penance
You can't stop your opponent from trampling over your High Priest this way. Although you are correct that trample damage is not assigned simultaneously between creatures and players, it is dealt simultaneously.


  510.2 Second, all combat damage that's been assigned is dealt simultaneously.


So both you and your High Priest are dealt damage at the same time. At that point, you may use your Priest's ability, but destroying the attacking creature or aura will have no effect on what already happened.