Pertussis (Whooping Cough): The patient was directly exposed to whooping cough at school, has been experiencing worsening cough symptoms over the past week, and the cough is worse at night, which are all classic symptoms of pertussis. The recent exposure and the timing of the symptoms strongly support this diagnosis.

Influenza: Given the time of year and the symptoms of a worsening cold and cough, influenza is a possible diagnosis, especially if the patient has not received a flu vaccine.
Common Cold: Although the symptoms have been worsening, a common cold could still be a consideration, especially if the patient has been experiencing other cold-like symptoms such as a runny nose or sore throat.
Asthma or Reactive Airway Disease: If the patient has a history of asthma or reactive airway disease, the cough could be exacerbated by environmental factors or viral infections.

Pneumonia: While less likely given the information, pneumonia is a serious condition that could present with worsening cough and cold symptoms. It's crucial to consider, especially if the patient has a fever, shortness of breath, or chest pain.
Tuberculosis (TB): Although rare in this context, TB is a critical diagnosis not to miss, especially if the patient has been exposed to someone with TB or has risk factors such as immunocompromised status.

Cystic Fibrosis: If the patient has a persistent and worsening cough without a clear infectious cause, cystic fibrosis could be considered, although it would be rare for this diagnosis to be made at 17 years old without prior symptoms.
Foreign Body Aspiration: If the cough is severe and acute, especially if associated with choking or difficulty breathing, foreign body aspiration should be considered, although it's less likely given the gradual worsening of symptoms over a week.