Paywalls are the single biggest practical obstacle to a high-school literature review. These four sources between them cover a large share of what you will need, legally and in full text:
- PMC (pmc.ncbi.nlm.nih.gov) — open-access biomedical and life-science literature.
- arXiv (arxiv.org) — preprints in physics, mathematics, computer science, and quantitative biology. Not peer reviewed: treat accordingly.
- DOAJ (doaj.org) — an index of vetted open-access journals across all fields.
- Your public or school library — many provide free remote access to subscription databases with a library card. This is consistently the most underused option.
If none of these work, email the corresponding author. A short, specific message asking for a copy of their paper has a surprisingly high success rate; researchers are allowed to share their own work and are usually pleased to be asked.