By far the most onerous possibility, but easier than you think!! There are a variety of programs that can handle this method. Finale & Sibelius are industry-standard programs, but require purchase. Noteflight and Flat are cloud-based and therefore available for access and editing on any device. Both are free for a number of scores but require subscription for full access. Musescore is completely free, but not cloud-based. Therefore any scores requiring editing are only accessible from the device where the score was created. Click on the video to see a demo of the free option, Musescore.
Built into MuseScore is a PDF-reading feature. If you have a scan of sheet music, museScore can read it.
The Sheet Music Scanner App allows you to take a picture with your phone and converts the photo into a midi or mxml file. Both are easily exported and read by a variety of music software programs.
Newzik for Education is new, but there is a PDF import capability with this platform, as well as the ability to mark up the music and insert play-along videos.
Of course, Smartmusic already has lots of scores already digitized. If you want to give your students access to something not already in the library, here are two ways to import music into Smartmusic for dissemination to your students.
The Hal Leonard library is available for purchase in PracticeFirst. However, if you want to provide students access to something not in their library, here is how to import music into PracticeFirst for dissemination to your students.
Noteflight - automatic export into Soundtrap for students to play/record with MIDI.
P.S. - It also converts in the other direction - automatically importing to Noteflight from Soundtrap!