Is my solo for the upcoming solo and ensemble and is apparently a very popular solo...Has anyone ever heard of it?

I wouldn't say popular, but it is played very often because of it's comfortable range and general accessibility to younger (freshman/sophomore) high school students. It was the first solo I played in High school, then I played Telemann Sonata in F, then Suite for Euphonium(Tuba), and then Introduction and Dance.

Really?its not popular but its played often?hmmm

You just have to remember that "popular" can mean a few things. Popular among early high schoolers may not necessarily mean poplular among the advanced players and professionals.
It doesn't matter really. Just play whatever you play with great intonation, time, musicality and style and you'll be just fine.

I love suite for tuba( euph) i did that one in highschool too.

Um any way yea the mozart is a good solo, you may not be the only one who plays it, but that doesnt matter. what matters is tht you are the one that plays it the way the judges want to hear it. see i didnt say the best, or most accurate that is because i played Bolero a couple years ago for an audition and technically it was ppractically perfect. The intonatin was excelent and the execution was brillient and my tone was really good ( that is the feed back i got that is not just my opinion) an older man played the same piece and the same arangement but he took a lot (LOT) more liberties with it and he got the seat.

Any way good luck