"Uh, Mr. Band Director?"
"Yes, Billy?"
"We just got these parts in for the new piece, and I have the wrong instrument."
"What are you talking about?"
"The part says Cornet I at the top. I play the trumpet."
"Oh, THAT. Don't worry, Billy, the fingerings and harmonics are the same... the two are basically interchangeable. You can just play your trumpet."
"No, they're not."
"Huh? Not what?"
"Not interchangeable. The cornet is conical, whereas the trumpet is cylindrical. It's also a little bit shorter in length than the trumpet... but I digress. The composer calls for a cornet, which has a less bright sound than the trumpet and is more suited to melody and harmony than fanfare."
"In fact, the two are very different playing experiences -- cornets often make the harmonics feel closer together than trumpets because of their design."
"Yes, you're right."
"Some composers will go so far as to call for both trumpets and cornets in their orchestra, and write extremely different parts for each..."
"That's all well and good, but the school district can't afford both cornets and trumpets, you know."
"Well, we ought to tell them how different the two can sound or something..."
"Billy, have a seat."
"I mean, you COULD use a trumpet in a cornet's place..."
"...but it just wouldn't be the same, you know?"
"Billy, have a seat before you have to play the sousaphone."

(SFX: rapid footsteps toward seat)