My daughter is 14 and is due to go on a school trip to France in 13 days rom the UK. We just realised that her UK passport is expired. But she also has a valid US passport. Can she use this? And if not, will we be able to get her a UK replacement passport in time?
I cannot think of a reason why a US citizen would be unable to travel to France from the UK with a school group using her US passport, even if she is also a British citizen.
To get back into the UK, however, she should bring some evidence of her British citizenship. The expired passport should suffice. See, for example