Cryptoy is a fun, free, educational app about cryptography, designed by GCHQ for use by secondary school students and their teachers.
The app enables users to understand basic encryption techniques, learn about their history and then have a go at creating their own encoded messages. These can then be shared with friends via social media or more traditional means and the recipients can use the app to try to decipher the messages.
For full details and to download the app go here: Cryptoy App
It is up to you whether or not you trust GCHQ enough to install it but it could be great for teens interested in cryptography.