Have the choir brainstorm music relating to current areas of study and present their ideas to the class. This can include songs that already exist or songs that they wish to write themselves.
Instruct all members of the class, not solely choir members, to participate in putting the songs together in some way. Hand out available instruments so that kids can help provide music for the singers, and have other children help come up with rhymes and lyrics.
Have kids make their own instruments if none or not enough are available. This can be through something as simple as placing rice or beads in a bottle to act as a shaker or mariachi.
Hold a concert in the classroom so that the students can present their song. You can judge them based on the content, its accuracy pertaining to the area of study and overall creativity.
Break kids up into groups for a different variation of this activity in which each group has a choir member, a musician and a song writer. Groups can then create individual songs and present them to each other.