1. Have you picked a language that your department has some existing knowledge and expertise in or can you get help and training
If it is a language that your team already know, then that gives you a big advantage or if that is not possible choose a language that you are able to get good quality support and resources for. Don't be afraid to reach out to your local Computing At Schools group for help or even to local industry and parents. They may not be willing to teach students, but they might help the teachers.
2. What are the teaching resources available for this language?
A good idea to check http://computingatschool.org.uk/ and the various book that are available for the language
3. Is the graphical and text language capable of allowing the students to model algorithmic solutions, teach concepts and principles you need to cover?
Visual Language
Scratch is a good choice, but also consider Kodu if you are in a Windows environment. If you want it to be possible to edit on an iPad Snap is a good cross platform alternative. Touchdevelop also looks interesting and resources are in development. For lower ability users the Hopscotch is becoming quite a fully featured package. Gamefroot and Blockly also provide possibilities.
Text-based language
The main choices for text based options are SmallBasic and its bigger cousin Visual Basic for Windows. (Or another version of Basic) Python is also popular and is good if you are planning to use the RaspberryPi. A language from MIT is Processing and it's got a lot of libraries and helpful resources. If you are in an Apple school, consider using Swift. You may also be able to use Javascript for a part of the curriculum.
4. Can pupils easily access the language at home and around school?
Think about access from home, cross platform languages are likely to be more accessible and web access is even better.
5. Will you be able to install the language on your network?
You will need the cooperation of your network managers to install most languages and there could be security risks to the network if improperly handled. So you can understand that there may be reservations. If not possible Web-based languages or Raspberry Pis provide possible workarounds.
6. What will you be teaching in Key Stage 4 and 5, is there progression?
It makes sense to teach a language that will develop into a useful language beyond Key Stage 3. Check your exam boards carefully to see which languages are suited and even where they say they accept a variety of languages, remember that the functionality of the language can make tasks easier or more difficult.
7. What have they learnt in Key Stage 2?
Certainly makes sense to use the visual language that is being taught in primary. (Or at least they are preparing to teach.)
Certainly makes sense to use the visual language that is being taught in primary. (Or at least they are preparing to teach.)