That must be a very large girl since that Tenor F-key trombone looks so small. Had F-key tubing and thumb valve not been present I would have guessed her to be holding an Alto trombone.
Upon closer inspection I see that there is nothing connecting the thumb valve to the tubing. It's even placed in the wrong area.
I do not know why she has a treble clef since the trombone she holds plays in the bass clef.
LOL if we were to dis every post with errors to musical instruments then 90% of K-on posts with Mio and her bass would be flagged... I own (cough) too many (possibly a number between 8 and 10) (cough) US/CIJ/MIJ Fender & Squire Jazz basses of various years and models and none of them look like frankly unplayably abstract "basses" Mio mostly gets drawn with.. but meh! I'm not expecting every artist t be a Fender fanboy