It sounds like a great idea.
How many days do you plan to spend on the festival?
What �s the aim of the festival?
What goals would you like to reach as a result?
Do you want to mix all ages?
What theme?
Competitions and concert sounds like fun. You �ve already got some nice ideas from Vickiii.
Do you think older kids can prepare something for little ones.
You can organise games similar to something popular on TV only with English stuffing.
Doing some exibitions is also a nice idea. You can put up some of the best works of your students or they can prepare something special for the event. Some students can act like guides. They can prepare info on British countries, famous cities, so on. I would not have used Harry Potter and similar characters for this event.
You can make it seasonal or dedicate to celebrations in general.
Would be nice to know how did it go!!!