What tunes are both challenging and achievable for beginning improvisers?
What tunes will support their self-efficacy, motivate them to continue to work at improvisation, and inspire them as musicians?
Consider whether the head of a tune suits your beginners (tonality, syncopation, etc.), but most of all, consider the HARMONIC RHYTHM of a tune's harmonic form:
Modal tunes are great because of their relatively slow harmonic rhythm; they feature large sections in which students can improvise over the same chord. In my experience, however, this is the same reason that students sometimes get lost in the form in a modal tune, especially if it's a 32-bar (AABA) tune. On top of that, the longer the form, the longer it takes for everyone in combo to take a chorus. You can break 32-bar tunes into their sections to make them more manageable, but balance that with putting them back together to make sure students work on hearing the changes and following the form. Highlight the contrast of the B section. In the end, if you're going to work on a modal tune with students, consider pairing it with another contrasting tune. This Google Slides presentation explains a modal harmonic form using John Coltrane's "Impressions" as an example.
12-bar blues are great for two reasons: (1) they sound and feel like jazz right away to many beginning improvisers, which is motivating, and (2) in my experience, students quickly learn to follow the 12-bar form, which promotes achievement and self-efficacy. Choose a typical 12-bar tune or one with substitution chords, like one that ends with the hallmark II-V-I chord progression. Some jazz educators will express concerns about the faster harmonic rhythm of a 12-bar harmonic form, especially compared to a modal tune. I have found, though, that the variety (as compared to a modal tune) actually assists beginners in hearing the changes, following the form, and staying engaged. If students do get lost, they can usually hear the turnaround and know that a new chorus is beginning. This Google Slides presentation explains a 12-bar blues harmonic form using Sonny Rollins' "Sonnymoon for Two" as an example.
KEEP AN OPEN MIND about other tunes.
Obviously don't throw "Rhythm Changes" in front of beginners, but if your students are interested in the head of a tune, and your rhythm section can swing the changes (pun intended), give it a try. For example, you might look at a tune like Benny Golson's "Killer Joe" and raise an eyebrow at the B section. It's doable, though, if you and your slightly more experienced beginner approach it with a strategy (e.g., choose specific chord tones ahead of time or practice variations on the original melody).
Promote variety. Not every tune has to be a blues, and not every tune has to swing. Choose tunes with straight rhythmic feels, including the great variety of Latin jazz.
If your students come to you with an appropriate pop/rock/hip-hop tune, embrace it. Your students can apply all of their jazz improvisation skills to a pop/rock/hip-hop tune. Tunes like that aren't replacing a jazz education, they're complementing it. When you show your students that you care about their interests, about their culture as adolescents, you build and strengthen your relationship with them.