Hate to be the bearer of bad news, but one month is hardly enough to build up a portfolio if you've never created games before (which i assume from your question), and especially so if you do not have programming experience (which is indicated by your question as well).
On the other hand I doubt that a game development school requires a portfolio of games. If you already had a portfolie of multiple games, what would be the point of attending this school anyways? It is reasonable for them to expect you to have some programming experience though, but even that might not be the case.
Three questions:
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Did you select these languages you tagged yourself, or are they prerequisites of the school?
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Is programming experience required by the school?
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Do you have programming experience or is this your first contact with programming?
If these languages are required by the school:
Unless you already have experience in programming, you are not going to be able to learn C++ AND C# AND Java in under a month, not even the basics. The most difficult part is not to learn the syntax of a specific language, but learning how to split a problem into such small parts that a computer can solve them.
Instead you should focus on ONE language first, getting accustomed to its syntax just enough to then learn the principles of programming, which apply across multiple languages (e.g. Object-oriented-programming works 'the same' for C++, C#, Java, Python and other languages that support it, nearly the only thing that changes is how you have to type it).
As Gnollrunner suggested, give C# a shot (used for example by Unity3D). Java is great for Android-development though, since it's the native language of Android.
If you chose these languages yourself:
Consider learning a simpler language, e.g. Javascript (programming industries biggest language currently, according to Stockoverflows 2017 survey at least) and use it to write simple web-based games.