Not in terms of using miles, but answering the more general ethical question might depend on the relationship you have with father and whether there's an agreement about him helping (or not) to pay for your school. For example, if he's supposed to give you $10,000 for school expenses and he now gives you the plant ticket in addition, there's an argument that his cash contribution should be reduced, perhaps by $500. It could also depend on how he might otherwise use his miles.