Secondary STEM resources
This page is a list of free (as in no payment required) resources for teaching STEM topics at the primary level (ages 12-18). Some may be under non-free licences.
Science
editScience Net Links
editFree lessons from the American Academic for the Advancement of Science. http://sciencenetlinks.com/lessons/
NASA
editPublic domain educational resources from NASA. https://www.nasa.gov/education/resources
Technology
editBootstrap
editFull curricula in using programming to learn, algebra, data science, physics, etc. http://www.bootstrapworld.org/materials/spring2018/
Engineering
editHacking STEM Library
editLower secondary (middle school) library of activities from Microsoft Education. https://www.microsoft.com/en-us/education/education-workshop/activity-library.aspx
Mathematics
editNRICH - enriching mathematics
editActivities for enriching maths learning by the University of Cambridge. https://nrich.maths.org/
Also has resources for primary school students.