I think it will make it even better! When you show universities how committed you are at what you love to do, they will be impressed! Especially when your putting yourself outside your comfort zone. It is hard being head girl, so when you actually take these responsibilities they will probably be more likely to accept you, as they want great people in their university. But you still have to work hard regardless of what you choose to do in this situation, so take the decision that is most appealing to you.