From CIMT who are one of my favourite sites for a reason – see this GCSE additional unit on Proof. A favourite site because if you are ever short of examples it is highly likely you will find something on CIMT who have everything from resources for little people to Advanced Level and everything in between!
Nrich have this collection Reasoning, Justifying, Convincing and Proof for Lower Secondary. A search on Nrich on Proof returns a wonderful selection for all ages and stages. We have tasks to introduce ideas of proof to younger children through to preparation for STEP examinations.
Jo Morgan (Resourceaholic) alerted us to a very interesting resource from the Institute of Education, University of London, a proof survey with some great questions for Year 10. Looking for more information about this, I found another for Year 9!
This was a lively chat with many contributions giving ideas and resources – the chat is clearly summarised as mentioned above on the NCETM website.