Hi Jamie! Noticed that no-one has come up with any ideas so I'll give it a go.....
The part of Spain I know best is the Costa del Sol, around the Estepona and Marbella end, and, depending on what your 14 to 15 year olds are into, I would say there's plenty to keep them amused - sailing, horse riding, Selwo Safari Park, etc., etc. (Perhaps they're a bit young for the fleshpots of Puerto Banus!!!)
However, when my son was 14 he went on a camping holiday with the school to the Costa Brava and loved every minute! From there you can also visit Barcelona - always a great pull! And, for my money, the coastline up there is far more beautiful than further south.
Hope that helps a bit.............