Both John Conde's and ryeballar's answers are correct and will work.
If you want to get too geeky:
John's has the downside that it has to make two decisions per $digest loop (it has to decide whether to add/remove center
and it has to decide whether to add/remove left
), when clearly only one is needed.
Ryeballar's relies on the ternary operator which is probably going to be removed at some point (because the view should not contain any logic). (We can't be sure it will indeed be removed and it probably won't be any time soon, but if there is a more "safe" solution, why not ?)
So, you can do the following as an alternative:
ng-class="{true:'center',false:'left'}[page.isSelected(1)]"