I have done over 100 murals with kids on our campus over 12 years. The
larger the number of students working together on one mural together the
tougher it is to orchestrate. If your campus has a mascot the environment
for the mascot gives you a good background. Then you incorperate evryones
best drawing into that environment. Say, Jenny is good at Butterflies, she
becomes the butterfly expert. She teaches and directs others on the location
and details of the butterflies in the scene. The "I Spy" concept is a hit on
our campus with visitors. They have a list of things to find in the jungle
scene. Find 7 panthers, two parrots, and 12 butterflies in our jungle. The
caption, "Welcome to The Jungle" is always fun.
The other suggestion is to break up the larger group into smaller more
manageable groups. Do a series of smaller murals with painted frames around
them. This is much easier to manage and instruct.