Luckily, I've already built in some mechanisms that could help them walk away with singed finger tips and a lesson learned, as opposed to third-degree burns. For one, I've made sure that each successive assignment is worth more than the last, which means that if they do poorly on the first few it will impact their grade, but still give them a chance to get back on track and hopefully learn from their mistakes. Also, students are required at several points in the semester to submit feedback on their group mates, which will influence the grades of individual members of the teams. In my experience, pressure from their peers can often lead to students improving their performance.
Although I hated having to give some of the teams harsh feedback this week, it gave me a boost to see how many students turned up in class today (the last Friday before Spring Break). The majority of students were present and met with their groups for at least half of the class period. I brought in materials (poster boards, paper, glue, etc.) for them to get started on their posters, and was impressed to see the students working together to plan their poster layout and calling me over to ask questions. I'm cautiously optimistic that the time I have taken to provide them with feedback - both on the process and the products of their team work - may not have been in vain after all.