Any easy STEM research articles/papers that high school students can understand?
Most recent papers are fairly hard to understand. After all, if they are published, it is because they present something that is not obvious.
You could however use older articles that describe phenomena that have gone on the high school curriculum since. For instance, here is a paper from 1800 generally viewed as the discovery of infrared radiation, which somewhat follows today's standard presentation of methods/results/analyses.