Tenor clef is correct for cello, bassoon, and trombone whenever their line rises into the upper-middle register

Tenor clef (C clef on the fourth line, so middle C sits on the staff's fourth line from the bottom) is the standard clef for cello, bassoon, and tenor trombone parts when the line's tessitura rises out of bass clef's comfortable range but does not yet justify treble clef — roughly, when a sustained passage would otherwise sit at 2+ ledger lines above the bass staff. This is exactly the case Sean named by name in the assignment brief: 'cello thumb position' and 'high bassoon' are the textbook triggers for a tenor-clef switch.

A cello passage in thumb position (sustained material from roughly C4 upward) engraved entirely in bass clef with 3+ ledger lines throughout, instead of switching to tenor (or treble, for the highest passages) clef.

Gould, Behind Bars, Clefs chapter, names cello, bassoon, and tenor trombone explicitly as the standard tenor-clef instruments and gives the same 'rises above the bass staff's comfortable range' trigger. Adler, The Study of Orchestration, gives the same convention with the specific register detail (cello: tenor clef typically from around G3-C4 upward through thumb-position writing; bassoon: tenor clef common from around middle-C upward). Ross and Read agree on the underlying principle without adding instrument-specific detail beyond Gould's.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.