I am surprised that you have not had any responses so far. Let me have a go.
Modernism, for the sake of our Extension History course, might refer to the blossoming of our intellectual heritage from the European Enlightenment. In simple terms, if we are rational, think in a scientific way and venerate the pursuit of knowledge then we will get close to the truth. The accumulation of knowledge/wisdom that results will ensure progress for civilisation.
Postmodernism suggests that we should be over this. In simple terms, the pursuit of 'objective truth' is illusory. All knowledge is subjective. All 'texts' are positioned and contingent. Nothing is authoriative and we should be very sceptical about the 'big stories' (eg the progress of Western Civilisation).
Two important points:
1. Postmodernism can seem to mean almost anything. 'Definitions' vary, depending upon whether we are talking about Architecture, Art, Literature etc
2. For History, postmodernism (at its extreme) challenges the idea that there can be any 'objective' reconstruction of the past. At one level, this might seem to undermine the whole history project that we are all engaged in. At another level, it is somewhat ho-hum. No intelligent historian would ever claim to have written the last word about the past and even the most conservative of 'traditional' historians would accept the role of debate and revision in history.
With Extension History, I do not think that secondary students should be overwhelmed with postmodernism or its significance, particularly in the forms that it is often presented.
However, I do think that students should be able to work around Keith Jenkins' definition:History consists of: 1. The Past. 2. Historiography (What historians do with the past.)
This distinction really is the starting point for any discussion in Extension History.
Keep an eye out for HTA's March Extension Day where Michael Hayes will offer two very good sessions on postmodernism. Program will go up on website as soon as it is finalised.
Also, it may be worth having a look at the journal index on this website for Marnie Hughes Warrington's recent journal articles on pm. She is very clear.