Ordinary student writing assignments may seem far removed from the concerns of Ethical Edtech. However, many of the tools used for both traditional and "digital" forms of student writing impose unethical forms of restriction and data collection processes on the user.
For ethical word processing software, consider the free software office suite LibreOffice.
For tools that support digital and collaborative writing pedagogies, consider some of the other tools on this list.
Danica Savonick's article "Why I Teach With HASTAC: Platforms as Critical Pedagogy" provides a good discussion about why tools matter in the writing classroom.