We were asked to purchase Sibelius perpetual (not ultimate perpetual) at our child's specialist music school (conservatory attached high school). That said, one of our instrumental teachers strongly prefers one of the free programs, MuseScore.

In particular it is useful for importing scores from online repositories to work from. For example when needing to transcribe a piece of music into the correct key for your instrument. It's a boring task to put the whole thing into Sibelius, then transcribe, or transcribe as you go. Not that long ago, when one of the other instrumental teachers was unable to source the correct version of a piece, and we could not source it at our local stores, this teacher found it online as a musescore file in the original key, transcribed it and sent it to me in moments. This was an old and well known piece, out of copyright. But written for a different instrument and not easily available in the correct key.