I know this post might be silly but, I want to ask if the word or better yet the question "what's?" can be used as interjection. Well, I'm asking this question. Because, I used to live in the U.S. and I remember that when I texted with friends they would sometimes reply; "Whats?" or "What's?" I never paid attention to it until now that I was chatting with a friend of mine who is an English teacher and she said to me it's not "What's" is just "What" the S and the apostrophy aren't needed but I told her. I wrote that as in "what's that?" she said that if that was the case then yes it was fine. But, that just the word "what's" by it self is just wrong. So here I am wondering if she is right and my friends back in the U.S. are wrong or if it's her who is wrong. Worth to say that my friends back in the U.S. are native speakers but my teacher friend isn't.
Uhm... you may have noticed that not all the rules of spelling, grammar, punctuation, and clear writing are followed when people text. :-)
I've never heard of "What's" used as an interjection, but that doesn't mean it hasn't developed in some sub-culture of sub-set of people. Language "fads" are particularly common among teenagers. (I'm just throwing this out; no idea how old your friends are.)
To answer your specific question, the English teacher is right. "What's" is not an interjection is standard English.
However, I won't say your friends are wrong. Language is about communication, and if your friends understand the meaning when you text them "What's?" then by all means use it. But beyond that circle of friends, I would not advise this usage.