I have to make sure that the lesson is hard. This is the final stage of my interview for a position in a language school. I have some on ' getting to know each other ' but they do not look that hard. I have to give a fifty minute demonstration lesson.

Try some of the sites listed at this link:
http://www.csun.edu/~hcedu013/eslplans.html#Lesson%20Plans

They all offer lesson plans and advice.

you could try the stuff in primary resources.co.uk its meant for primary school children but there are so many ideas there maybe it will help and you could use some of them by adjusting them

I think Dave's ESL Cafe might still have a link.

I suggest onestopenglish.com and Macmillan resource sites. However, as you will be dealing with other ESL professionals who are likely to know what resources are available online, I advise you not to follow a lesson plan to the letter but to gather ideas and activities to elaborate your own.

Do you know what book upper-intermediate or advanced students in that school use? That could give you an idea of the level they are aiming for; and if you choose to use such books, it could also give them an idea of what you are capable of doing with their adopted material.