I got this comment after more than 75 days of submission:
The considered problem in this manuscript is a practice in high mathematics and can not be a paper for publication in high-level journals.
What does it mean?
My educated guess is that the reviewer is a native speaker of German and mistranslated some terms into English:
The German word Übung translates both to practice and to exercise.
In German universities, höhere Mathematik (literally: higher mathematics) was used as a label for mathematics courses for students of other scientific fields or engineering. At times, this only referred to the calculus courses (and not the linear-algebra courses). In particular, these courses usually focus on applying mathematical concepts and do not compare to courses for students of mathematics. (I wouldn’t be surprised if you can still find this label in use somewhere.)
If my guess is correct, the reviewers comment means something along the lines of:
This manuscript is just a (demonstrative) exercise in undergraduate mathematics and therefore not suitable for publication in a high-level journal.
So, your manuscript was rejected because it allegedly does not feature any new mathematics.
Seems to me that there's a missing word, and the correct comment might be something more like:
The considered problem in this manuscript is a practice in high school mathematics and can not be a paper for publication in high-level journals.
In other words, your manuscript is too simple to be published in an academic journal.
The considered problem in this manuscript is a practice in high mathematics...
In context, I would translate this into native English as:
In higher mathematics, the problem considered in this manuscript is just an exercise...
In other words, a routine calculation that has nothing new to say. Sorry.
This seems really odd. Is this the journal you submitted to?
It took 75 days to receive a 1 sentence answer in English that makes no sense.
Even if it was high-school level, any peer reviewer would be expected to write more than a single bad English sentence.
There should be multiple reviews, and the editor should have sent a single sentence review back for more details without being asked.
What to do
If you are a grad student, take this to your advisor. There is a good chance they can contact the editor, and will have more clout than you (unless you are a professor).
If you are affiliated with a university, ask others in the field about it. If there are multiple people with single sentence reviews, push back on the conference as a group.
At least reach out to the editor. Say
I'm not sure what this sentence means. This is the only review I received from my submission, and I'm not sure what it means or why my paper was rejected.
I thought it might be a desk rejection, but the message isn't boilerplate. I'm also not convinced the review meant "high-school".
If it was a high-school caliber entry, and the editor caught it, why not send a form letter or ignore it?
I have a difficult time seeing how "high-school math" can be anything but an insult. Why not just use words like stupid outright?
The editors are implying that what you have reported in your manuscript is already out there. It could be a theorem or a technique already in use by mathematicians, and therefore cannot be published as your research finding now.
The considered problem in this manuscript is a practice in high mathematics and can not be a paper for publication in high-level journals
Probably he means "high school". But, Gauss found many solutions in his high school years... Nobody exactly defined where "high school" ends. Because in my high school, I modelled Frank-Hertz experiment. With complicated enough approach this could become Plasma Physics article, enough for publication in any scientific grade journal.
Another probability is that they think subject lies in field of "high mathematics", and they understand it as "mathematical analysis". It exists in some high school programs, but generally considered university discipline, rather then school. Again, it is actually possible to create problem inside mathematical analysis and solve it, creating material of grade which is enough for publication anywhere.
Also you can always publish it in journals for PhD/high school students, which have lower bounds for inclusion of articles.
Not explaining what exactly in your result did not fit into bounds for publication is nonsense. Such uncompetent staff which doesn't even bother to explain the subject of article should be fired because most probably their PhD degree was bought (remember how french physicists had bought degrees in 90s and after great scandal they were both fired and their works disqualified).