There is no absolute requirement that data used in a study should have been specifically collected for that study. However, there are two important caveats that should be considered. From a methodological perspective, any study is based around one or more research questions, or some similar focus, and ideally details of the methodology should be designed accordingly. This includes such matters as the type of data to be collected, the amount of data needed, and the approach to sampling. These steps are all short-circuited when the data is re-used for a different purpose. It is important therefore to be confident that a data set is appropriate for use for the new purpose. Re-using a data set in this way probably means some compromise over research design features, but does provide considerable savings of time and effort!
Question for reflection
How much detail of the purpose of classroom observations should a researcher reveal in advance in order to obtain informed consent from teachers?
Balancing the methodological and the ethical imperatives
From an ethical perspective, data that is collected in schools is a gift to feelings, ideas, teachers and pupils. These individuals are the original 'owners' of their permit for beliefs, utterances, etc. and, as suggested above, a researcher must get consent for the use of such material in research