Noteflight is the best all-round option for K students, and especially for middle school students. There are some problems like no swing functionalities and the fact that triplets at the end of the measure and possible unless you change the time signature.
None are perfect but they all work very well! There is a bit of a learning curve to it. The software would for the life of me not select the small A at the right of the text to allow me to format. The interface is intuitive and easy to use.
Notion comes with a limited selection of sounds and you can access a wider range of extra sounds via in-app purchases. I tested it with my 7 year-old some time ago link to previous post and you can see the results in this Using NotateMe with Students video: US teacher Christopher Russell has been using NotateMe with his middle school students in their music theory classes.
I write etudes for students, and use it for transcribing, transposing, or arranging songs. Little things like that. This video takes you through the basics of entering notes into your score: Transferring scores between music notation apps is done by importing and exporting scores in the MusicXML format — the standard open format for exchanging notation files between different musical applications.