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He didn't say he was in middle school, he said he was a junior in high school...which is easily mistaken for a being in "junior high school." "In" is such a pointless word anyways.

I am in almost the same predicament that you are, (I'm a junior, trying out for a concerto comp., except I don't have to memorize the piece.) It may be kind of taxing to memorize, but you could try out the piece I'm using, the Ewazen Concerto for Bass Trombone, (or Tuba) It's pretty great, and gives the soloist plenty of moments to "shine,", (or, in my case, be dull, as my horn is raw brass and does not shine that much light.)

I you are playing a solo, you should know the music well enough to have it memorized. Whether you perform it from memory is another question entirely.

I performed one concerto with the symphony with the music on a stand beside me. I didn't use it, but I didn't worry about having any memory slips, either...