After having done this for many years, I think I've finally streamlined this and have it down! This can be done in one 50 minute class if you are prepared, but more time is better. The yarn sits in the sun in those baggies for days (I have the students put them out in the morning, and bring back in at the end of the school day). The next class we have in a week, we will rinse and hang the yarn to dry.
Have I mentioned I have 2 fifth grades, both at about 30 students...that's a lot of dyeing!