Some teachers have a practice of making a big deal out of making it known which students got high scores on assessments or assignments that they had done. They might pass out the assignment and say "So-and-so got a perfect score" or "So-and-so got a 9 out of 10." The great thing about that is that the students who you praise feed off of that recognition. It boosts their spirits.
However, what about those students who are not praised due to the score that they had gotten? Necessarily, if you only recognize the students who do well, then you will not recognize the students who do not perform at the ability that you define as well. In which case, you would be willingly not recognizing their effort. What kind of message does that convey? It conveys the message: "My work is not good enough to be recognized." As such, that kind of lack of recognition in light of those students who are given recognition basically serves as a put down.
So, what do you do then? It seems undeniable that you need to be equitable in how you distribute your recognition. So, it sounds like your recognition must be all or nothing. Either you recognize everyone's efforts regardless of their score or you recognize no one's efforts. If you recognize no one's effort, then everyone will feel undervalued. So, it seems then that somehow, you must recognize everyone's efforts. How do you do that?
I can't give you a specific method per se, but you must be able to recognize when anyone does good. That's how you make a person feel valued or like they are accomplishing something. One way in which students serve students are as gauges of their own success. I say that because that's how I felt when I was working on my undergrad. I fought hard for the approval of my Graduate Student Instructors. Whatever positive approval I got from them made me feel like I was progressing. I think it's the same way for students and teachers in elementary school. Whatever positive feedback can be given will be motivating to them. But, simply neglecting a students efforts because they didn't do well will make them feel undervalued. Hence, that will demotivate them.