If you want research-level physics papers about topics high school students can understand, your best bet might be to look to the past. Older papers are great fun to read, but with their archaic language and notation they're not always the most efficient way to learn.

One famous exception is Einstein's 1905 classic On the Electrodynamics of Moving Bodies. This is where special relativity was first introduced to the world. A high school student might struggle with the second half, once electrodynamics comes into play, but the first 12 pages or so are mathematically simple, and a rewarding read. If they can get through the whole thing, however, they'll be rewarded with a little bonus addendum, which derives a certain famous $E=mc^2$.

In thermodynamics, Sadi Carnot's Reflections on the Motive Power of Heat is quite readable, and Joule's On the Motive Power of Heat is fascinating. That said, neither of these authors write like Einstein, and there are better places to learn thermodynamics today.