"Because you and your friends were too busy to meet up with one of your mutural friends from another school last night, she emailed you wondering why all of you stood her up. Return her email and (1) apoligize for not calling her to let her know that you and your friends could not make it. (2)....." the rest is pretty easy.
How would I word the apolgizing for not calling part? Spanish isn't my strong point. All I could come up with is "lo siento."