avg_start_date = Students.objects.filter(school_passed=True,
    started_college=True).aggregate(Avg('school_start_date'))
avg_passout_date = Students.objects.filter(school_passed=True,
    started_college=True).aggregate(Avg('school_passout_date'))
avg_time_at_college = avg_passout_date - avg_start_date
Django ORM вычисляет количество дней между двумя атрибутами данных