The quickest and cheapest way to get her home by tomorrow, would be by plane. The following site, below, lists the airlines which fly between the UK and Alicante Airport, the closest to Benidorm; she would have to take a car service or airport shuttle from Benidorm to Alicante Airport and she should not go unaccompanied. I believe children who fly, under the age of 12, unaccompanied by an adult, need to be registered as such, with the airline, and a 'responsible' adult needs to put her on the plane and your name will be on a list, of who would be picking her up, so she will be released only to you, when she lands, upon verification of your identification.
If you can supply more specific information, like which airport she left from, and which she flew into (if not Alicante), to get to Benidorm, I can try to find you some reasonably priced flights.
It would be a good idea to speak to the adult in charge of her and her friends, to find out exactly what the problem is, and if perhaps she can stay on holiday for the pre-determined amount of time, and return as scheduled. If she sounded very distressed and there is no way she can stay for the entire time, I agree it's best to get her home as soon as possible. If it's feasible for you (or any other trusted friend, relative, etc.) to travel to Benidorm to pick her up and bring her home, this would be the best and safest idea, if you're able to swing it financially, and time-wise, re: work, etc.