First, let me congratulate you for your leadership ability and willingness to mentor less experienced players! That is a wonderful attribute. The beginning of my life as a teacher happened much the same way: as a teenager giving lessons to less experienced players. Bravo to you!
Your comment about prioritizing and how to work through them will be dependent on where the students are and the rate that they grasp the information. In other words, tailoring to the needs of the students is a great thing. Have the students make a list (AND YOU TOO) of what skills they want/need to acquire.
It is imperative to stress the importance of listening to the music one is wanting to play. Without listening, the conception will never happen.
When I was coming up, Rufus Reid's "Evolving Bassist" was extremely influential to my conception. The book has been revised and is still relevant and one of the most important tools. There is also a DVD. Start with that and you cannot go wrong. That book has helped so many people!
John Goldsby's "The Jazz Bass Book" has become another essential tool for bassists.
I will make comments on your action items point by point.
You said: "To be a bit more specific, I need to cover:"
- improvised walking lines
Many method books begin with or include somewhere a discussion of using lower chromatic approach tones to approach the downbeat of the next bar with a root of that chord. Using a blues is quite useful for this exercise. That is a great way to help the student understand how TO a place and think ahead about where the line is going. Transcribing easy bass lines is a great tool for understanding. Make sure the bass is well recorded. It is also helpful to look at pre-transcribed basslines and analyze how the lines work. Have them play along with recordings just for fun so they can get a feel for what a great groove is with swinging records.
- improvised bossa/Latin lines Oscar Stagnaro has a wonderful book "The Latin Bass Book". Playing along with classic recordings and transcription are also essential here.
- solos, eventually
Once again, listening to and playing along with classic recordings, reading pre-transcribes solos, and doing your own transcriptions is also essential here. Start with something simple! Starting out with Charlie Parker is just too frustrating! Please see below for the answer to the theory question because they are related.
- sight reading
Playing melodies from books, etudes, pre-transcribed solos, and basslines is great. For electric bass, Ed Friedland's "Hal Leonard Bass book 1" is pretty universal.
- the theory behind chords, scales, and modes, and how each apply to jazz
At the University of North Texas, we use "Building a Jazz Vocabulary" by Mike Steinel and "The Jazz Language" by Dan Haerle.
MANY MORE books are available from Jamey Aebersold.
http://www.jazzbooks.com/
Have them attend a summer workshop: We have two summer things for jazz bassists at the University of North Texas: a jazz double bass workshop at UNT, and a combo workshop. (I had to mention those!)
http://jazz.unt.edu/workshops
I also teach at Jamey Aebersold's Jazz Workshop which set the gold standard for summer jazz camps!
http://www.jazzbooks.com/
Hopefully, that helps!