From the National Archives including primary sources and online teaching activities:
https://education.blogs.archives.gov/2020/11/09/wwi-resources/
From the Library of Congress:
The Library of Congress offers classroom materials and professional development to help teachers effectively use primary sources from the Library's vast digital collections.
https://www.loc.gov/classroom-materials/world-war-i/
https://www.loc.gov/classroom-materials/world-war-i-what-are-we-fighting-for-over-there/
From the National WWI Museum and Memorial:
From the BBC:
History videos, quizzes and activities that will help students practice their World War I knowledge and skills.
In commemoration of the 100th anniversary of World War I the National History Day Project created this guide:
https://images.history.com/images/media/pdf/history-classroom-WWIGuide.pdf
From the Open Educational Resources Project:
Curricula created by teachers and scholars that offer complete social studies courses aligned to state standards, all totally free. Site includes teaching materials, planning resources, professional-development offerings, and a helpful online teacher community.
These lessons and resources, which explore World War I’s profound political, economic, and social consequences, will help turn this complex period into a manageable narrative students will understand.