This lesson plan, Community Canvas, builds off the work of an Atlanta based teaching artists, Karin Mervis.  Her visual arts murals and whole-school art projects are inspiring in this time of social distance and virtual learning.
I adapted some of this and thought about how to do whole-school composition.  
Why I love this lesson:

• Great to involve the whole school or studio. 
• Easy entry into participation:several methods make it accessible to all.
• Very visual: put together a large composition as a mural, and for classroom decoration.
• Relevant: nice to have a composition activity that links to the arts, involves creative decisions and builds community

​Fellow teaching-artist Karin Mervis, based here in Atlanta, shared her
community canvas work she did at a local elementary (primary) school. She
says:
“25 years later... whilst still loving the method of creating Community Canvases with any
population ... in this case 5th grade ‘farewell’ legacy piece... I have learnt along the way to “look
closer”... “ Hone-in” on best practices and to bring more meaning to the moments with each
student... in this case the sky pieces were done as collaborations... finding a common symbol/ logo
for each team to include, that represented a part of their school creed:)))
The ‘self-portrait’ people were created by brainstorming ‘how to make the world a better place’ and
what mattered individually to each student... they then re- interpreted their findings into symbolic
representations of line color shape and pattern. Each
‘person’ on the painting has specific meaning; the students went above and beyond...!”

Let me highlight a few here so you can make a choice
as to the best fit for your situation, program, school or studio.

1. Secret Codes
https://www.tes.com/teaching-resource/secret-codes-music-
composition-lesson-11723170
This idea focuses on having a student use the musical letters of their name to generate a small 3 to 4 note motif. Use this idea and have this piece of “self-identity” as
building blocks for a large-scale work. The musical letters can also
spell out other aspects of belonging, community, group identity, etc.
Layer, align, stagger, harmonize, or link-up all these motifs. Glue
down onto large canvas to have a display for website or school site.

2. Virtual Choir by Eric Whitacre
https://ericwhitacre.com/the-virtual-choir
Those of you who have attended my presentations know that I feature
this idea as a more advanced project for large groups. This does
require more advanced skills and a big tech effort, but we do have
schools and teachers who have done similar projects.

3. Very Variation
https://www.tes.com/teaching-resource/very-variation-music-
composition-lesson-plan-11723849
This lesson explores variation form and is an obvious fit for expanding basic motifs. Students could compose their very basic self-identifying motif and as a build-on this could be passed up along higher classes to compose variations. For example, first
grade composes the basic motif, second and third grades set those
motifs to rhythm, fourth and fifth grade compose a variation (weaving
in their own self-identifying motif), sixth and seventh grades compose
another variation, and etc. All of these are then assembled school-
wide through linking all contributions into one large composition for
display or performance.

4. Chord Cards
https://www.tes.com/teaching-resource/chord-cards-music-
composition-lesson-plan-12274092
A very simple approach to building motifs and composing are to
provide small building blocks, i.e., chord cards. Each class could have
one chord, e.g., 1st grade has CEG, 2nd grade has DFA, 3rd grade has
FAC, etc. Within the grade these three notes are used at their level of
rhythm writing to create short melodic fragments. Link the fragments
so each grade has a melodic phrase in that particular harmony. Then
link the chord-based melodic fragments school-wide in a harmonic
design. For example:

​You can see how we even have a modulation to the dominant major!

5. Word Painting
https://www.tes.com/teaching-resource/word-painting-music-
composition-lesson-plan-12273953
Word Painting provides a more advance level of composing using
words/lyrics to make a composition. This is an option for school
wide composition as well as each class or grade level could have a
topic or verse to contribute towards a large-scale piece. Some groups
could simply compose the introduction, others a rhythmic
accompaniment, and more advanced classes could compose the
melodies set to words. This is also a project that could work well with
your Language Arts classes in Arts Integrated schools. Students could
write the lyrics/verse in LA and could bring this over to music class to
explore setting words to music using word painting.

There are many more options using a basic music composition lesson plan and
figuring out to link, combine or layer them.