It is important to acknowledge that scheduling is often a barrier for music educators. For music educators who are interested forming jazz combos to teach beginning jazz improvisation, but who are concerned about scheduling limitations, consider some options:
If you already run pull-out lessons...
Consider designating one or more of your lesson slots as "jazz combo," "jazz improvisation," or "enrichment" slots. Choose a name that you think fits the focus of the class, or will garner the most interest, or will make the most sense to other stakeholders (administration, parents, students).
For students who already take like-instrument pull-out lessons, consider jazz combo as a second lesson.
If there are concerns about the additional pull-out time of a second lesson, consider rotating students into combos for one marking period at a time (e.g., 1 MP in a jazz combo, 3 in a like-instrument lesson). This will likely affect horn players the most; for rhythm section players, jazz combo may be their only lesson*).
If you already run a big band...
Consider designating some of a single rehearsal time as jazz combo time (e.g., split an hour rehearsal 40/20, an hour and a half 60/30).
If you have more than one rehearsal a week, consider doing the same as above, or designating one as big band time and another as combo time.
Run one combo in whatever time you designate, or run a rotation of multiple combos (alternate weeks for two combos, or run one a marking period for more).
If you already run sectionals, consider making one of the sectionals a jazz combo instead.
Designate times of year as "big band" or "combo" times (e.g., for a middle school, run big band Sept.-Dec. and Apr.-Jun., and combo Jan.-Mar.; for a high school competitive jazz program, run combo or big band in the fall, big band in competition season, and combo post-season)
If you are concerned about the big band rehearsal time you will "lose," consider the musical independence and jazz improvisation skills your students will gain. Start a combo as a project, or "experiment." Set a designated time for the "experiment," predict what benefits you hope to see as a result, and evaluate at a certain point whether you'd like to continue, or continue with changes.
If your school supports compensated music outside of the school day, and by some miracle you're not already overbooked with extra rehearsals...
Propose a jazz combo that meets before school, after school, or during a lunch/recess time; turn a time when many band students are "hanging out" into a meaningful music-making experience.
Start with one combo, and if interest expands, approach your administration about options to meet as a class.
If your school runs summer enrichment programs...
Propose jazz combo as an offering.
*In my experience, jazz combo opens a door for rhythm section players who have not previously enrolled in school music. These players make music friends and become interested in school music through jazz combo. Often they decide to learn a wind instrument and join another ensemble, or sign up for another music elective at school.