Mix grass seed in with potting soil. Fill up a nylon. Tie into segments with string to make a 'caterpillar." Soak in water till saturated. Then let the kids spray it with water every day after and watch it grow. When the grass gets "too long" let the kids give it a "haircut."
If you have classroom pets, it's best to have students and parents sign a classroom pet permission slip at the start of the year. Students often want to hold and care for certain types of pets and to ensure parent permission and student understanding of rules, this permission slip will keep you covered.