I am a high school student who has worked on multiple (well, 4 to be
exact) pure math and CS papers. All of them deal with fairly
significant problems, so some of them have been published by now too.
Wow. That's awesome!
Today, I saw a paper which looked quite similar to a paper that I
published back in May. I was shocked to notice that the author of that
paper had plagiarised my entire paper verbatim. [...] I am worried because [...] these papers
mean a lot to me as I have worked so hard on them.
Yes, of course. Trust me, there is no one who has ever written a serious paper who isn't just as emotionally invested in it as you are in your own papers, so we understand. I personally discovered only a few weeks ago that some loser plagiarized some of my work and submitted it as his own (not to a journal but as a final project for a university class, fortunately), so I know quite what an annoying and hurtful feeling it is, even though in my particular case this action does not threaten me in any way.
I have already notified the journal but I doubt that I will get a
reply. Could you please let me know the further steps to be taken too?
I agree with your assessment. Junk journals, like the people who publish in them, have no ethics, they are predatory entities usually based in corrupt and lawless countries. The "editor" is most likely a nephew of someone at the criminal or semi-legal enterprise that set up this journal.
As for what you should do, there aren't that many options that do not involve legal action of some sort. Basically, you should try to discredit the journal and the plagiarist (who are both complicit in this misconduct), and shame them into removing the plagiarized paper from the web, by contacting:
The department chair and other administrators (dean, ethics committee, ombudsperson) at the plagiarist's institution. Note that chances are this institution may itself be a very low-quality, poorly ranked and possibly corrupt institution in a less-than-developed country, so it is just as uncertain that your complaint will lead somewhere as in the case of complaining to the journal; but regardless, it certainly won't hurt to try.
Any journal indexing service or other similar entity that gives credibility to academic journals by including them on lists it compiles, and may not be aware of the scamminess of the journal in question. Here is an example of such a service that I found through Google (which also has a list of excluded journals that apparently it considers disreputable). I do not use such services myself so do not know which ones are important/credible and which ones aren't; perhaps others can comment on this.
Any news media in the country where the journal and/or plagiarist are based who may be interested in this story of academic fraud.
Western bloggers or journalists who are interested in dishonesty and fraud in academia. Again, I'm far from an expert on this so don't have too many specific suggestions, but Retraction Watch is a name that gets thrown around a lot.
At the end of the day, if all of those methods fail, your last resort would be legal action, since the journal and plagiarist author are committing an infringement of your copyright. Even if they are in far away countries, they are probably within the reach of the law, but it would likely be a long and costly battle, and I think you are right to want to avoid taking that route. Good luck, and keep up the good work! I'm sorry you are getting to experience the nasty part of academia at such an early stage before getting a chance to see all the good parts. I am sure this experience will not be representative and will be followed by many more positive ones.