That is a really tough environment. That size group is larger than anything I have experienced.
Is there any way you could 1) break them into two groups 2) get a large band room?
Since there are so many, open practice time might lead to chaos, so leading a group activity would probably work best.
My best advice would be to spend time working on melodies played on a single string - Happy Birthday, (see the table of contents of my Guitar Essentials Level One for song titles) etc. Teach them by singing the fret numbers and having them echo you.
They are almost too young for chords, so working on the melodies accomplishes several objectives (see Teaching Classroom Guitar published by MENC). When you are ready to read music - you may want to use tablature first since it will get things moving faster (See guitar essentials contents).
If you feel adventurous, you could remove all the strings, but the first string, and have them master the "monochord" - this is the name of the instrument that Pythagoras did his experiments on. It means "single string".
If you leave all the strings on you should be able to do "string percussion" exercises such as echo imitation etc.
Stay with each song until it is mastered. The students will benefit from "over learning" the piece.