Scaffolds for ensemble roles remove barriers that might prevent student musicians from performing their "jobs" in the jazz combo. They are especially important for rhythm section players, who are generally performing different tasks than they do in other parts of their musical lives. Eventually you will want players to learn these tasks on their own, like building and voicing chords or bass lines. Until then, scaffold the non-salient parts of the tasks so that players can fully engage in the ensemble.
Suggested chord voicings for guitar and piano players (make your own, or choose tunes from resources that already provide them, like "The Real Easy Book")
Suggested bass lines for bass players (make your own, or choose tunes from resources that already provide them, like "The Real Easy Book")
Root note bass lines for bass players (these are not necessarily something you need to notate; you may simply want to encourage a novice bass player, especially one who is struggling with ensemble cohesiveness, to play quarter note root notes; root notes are also a great place to start for bass players who are starting to compose their own bass lines, starting with roots, then roots and approach notes, then filling in with other chord tones)
The "T" word...TAB. Regardless of your opinion on tab in general, consider what is most important for novice bass players who have just joined combo, and who have only ever read tab. Reading music is on the list, but not before other skills (e.g., keeping a steady pulse, following a lead sheet, improvising, etc.). Consider getting them started with playing root note quarter notes, either by ear or by interpreting the chord symbols on the lead sheet. That will get them playing right away and familiarize them with the location of some recurring roots (e.g., Bb, Eb, F, C, G). Then, when the student does start to read, those notes will act as a reference. In the meantime, tab out suggested walking bass lines so that your new bass players can get the feeling of walking under their fingers.