Access millions of publications and connect with 15+ million researchers for free! Though ResearchGate started as a social networking site for scientists and researchers, the articles are not restricted to those on science topics (Wikipedia article).
"Academia.edu is a for-profit open repository of academic articles free to read by visitors (Wikipedia)."
A number of databases, which normally one would need to pay to access, offer Open Access journals and other resources, which would benefit those students in the upper years of secondary. This page will be dedicated to those resources. It may be easier to search these using Google Scholar.
Nature Portfolio is excellent for research in various areas of the sciences. Use their search bar for finding journal articles and then just look at those marked OPEN ACCESS. You will find the search bar and advanced search by clicking on SEARCH in the righthand top corner. Click on the image below to go to NATURE.