School science curricula habitually encourages students to develop science knowledge alongside 'ethical understanding', the moral theory of right and wrong. Drawing from the ideas of Karen Barad, Donna Haraway and Rosi Braidotti, this paper critically examines the 'science-ethics nexus' in Australian secondary schooling. In doing so, it offers a renewed definition of ethics that negates humanism's anthropocentrism and problematises humanist dualisms by centring a relational world. While posthumanism and new material feminism directly and critically engages with science and ethics, there is limited attention to how it might renew our understanding of these ideas in schools. Thus, the research question driving this study is, "How does posthumanism help renew thinking about the science-ethics nexus in secondary schooling?" A thinking-feeling-doing diffractive practice is utilised to analyse two data sources: the Science and Ethical Understanding streams within the Years 7-10 Australian Curriculum and a speculative short story by the first author, "The Beforetimer." 'Plugging in' concepts of posthumanist ethics and relationality, this alternative methodological approach mobilises the power of diffractive forces to help illuminate how the science-ethics nexus in schools can reproduce onto-epistemological traditions of 'Euro-Western' cultural and masculinist hegemony. This hegemony restricts diverse cultural approaches to understanding the science-ethics nexus by (a) giving credence to reason, as divorced from emotions, and objectivity; (b) defining ethics as separate from knowledge production; and (c) undermining ethical responsibility by merely associating it with specific topics/issues. In contrast, integrating posthumanist ethics in school science requires those implicated to explicitly attend to cultural hegemony and relationality and to place ethics at the centre of knowledge production. Pursuing a posthumanist ethics in science education must do more than critique anthropocentrism/humanism broadly but also interrogate and unsettle this Euro-Western hegemony over diverse ways-of-knowing-being.