Am I correct in assuming this is a theory for non-majors course of some kind?
If that is the case, you're going to have a huge variance in the amount of reading ability, from none at all to students who could have been music majors if they had chosen to. However, everyone is probably going to have some similar intrinsic knowledge about the aural aspect of music, simply because we live in a culture dominated by certain musical tendencies.
For this reason, the aural skills training you do will likely be more advanced than the theory at the start, and you should take advantage of this to scaffold the notation aspect until it gets up to speed.
(Oh, and I've never used it personally, but this textbook seems like it might be quite useful for what you're doing.)
I think a good goal for that age of student by the end of 24 sessions would be singing and transcribing simple melodies in a few common keys through the use of movable-Do solfege. The first step would be familiarizing the students with the diatonic solfege syllables, learning some common melodic patterns, and listening for how tonic and dominant work in some real music.
You can do the visual aspect of pitch contour without even having addressed key signature or clef yet—simply point out which line or space is "do" for any given example, or draw attention to the last note of a melody. Changing keys often will prevent students from getting too used to "do" always being middle C.
Overall, here's my philosophy: Take into account the three main pillars of music theory: rhythm (including meter), pitch (including harmony), and notation (including note names). You should identify the baseline for your students, and then work on all three concurrently. Notation will probably be built up from the absolute ground floor, but the other two pillars should be used to scaffold that one and each other, eventually bringing everything up to the same level while still moving forward in each subject.
Oh, and sing everything. Unless they're practicing penmanship (which, indeed, they should do: "draw 10 treble clefs on the next line"), find some way to link everything to a verbalization, rhythmatization, or vocalization.