£90k in April 2011 is just under £58k after tax so it would be difficult for most families on that income to afford a £30k boarding school. Other factors such as number of kids and if they are in private education are often factored in too. I remember one of the bursaries we were given was crudely based on taking our income, deduct 70% and if the remainder 30% covered school fees then no bursary would be offered. We had no assets so that wasn't taken into account.
To the OP, every school has a different policy.EDIT: I should add that the £90k scenario was a friends, not ours!