Germinating plants on a sponge

[RenettaHogan]

I'm getting ready to start a hydroponic unit with 6 classes of elementary students. Since funds are tight, is it possible to use sponges soaked in nutrient for the germination phase or is there an even cheaper way? Can I also use 20-20-20 or 20-15-20 fertilizer that contains micronutrients but add a little more epsom salt and calcium nitrate to the mix?