I second the poster session idea – I’ve done that at the end of a lot of my projects and it’s definitely better than whole class presentations. One suggestion to add, addressing your concern with helping the listening students be more active, is that whenever I’ve done poster sessions the listeners have had feedback sheets that they fill out and which get distributed to the presenters afterwards. In my cases, the feedback sheets have always been the same as the grading rubric that the professors use, but they are also significantly more bare-bones than yours: three categories and a 1 to 5 (or 1 to 7) scale with both extremes labeled. I think something like that provides a good amount of structure for students to think about how to evaluate each others’ work and provides a push of extra incentive to pay attention and really think about what’s being presented. (Added bonus: repeatedly doing this with similar rubrics over multiple projects has led me to better evaulate my own work by the same criteria while I’m still in the midst of the project and thus have time to fix it. No guarentee that it will have that effect on everyone, but it did for me at least.)

I think project proposals are a good idea. I’ve found that they can sometimes still be frustrating because it is totally possible for students to write one while still really having no idea what they’re going to do, but overall they’re still probably better than nothing. It’ll force all students to have at least planned a little, and will probably be significantly helpful to a decent fraction of the class. Along the same lines, I think a midpoint deadline is probably also a good idea. Honestly, you can probably be as vague as just telling them they have to produce a “deliverable,” and let them figure out what is most appropriate for their particular project – basically any form of preliminary results or a tangible demonstration of work in progress. If you want to be really ambitious about encouraging real project planning you can even tell them to write in their project proposal what their intermediate deliverable is going to be. (But if you do that, definitely make it clear that they’re not firmly committed to what they write – if their project starts going off in an unexpected direction they should be able to adjust to that and change their minds)

Another thing which I’ve found very helpful in my own projects is to declare at the beginning (in the project proposal) both a “minimum deliverable” and a more ambitious goal. Basically, the “minimum deliverable” is the simplest possible form of what you want to do, something that should be easily within reach. And the more ambitious goal is what you actually want to accomplish. Writing both helps me to figure out how to start with something simple and then build up from there once it works, which is infintely more productive than accidentally jumping in way over my head and then trying to figure out how to back out of that to something simpler. Essentially, writing down a tiered set of goals means that I’ve created my own scaffolding for the project, which makes it way easier to make constant steady progress and always know what the next step should be. I’m not sure how relevant that idea is going to be in your context, but it’s been so important to my projects’ successes that I thought I should at least throw the idea out there and you can do what you want with it.

Sorry this comment has gotten so long, but one final thought, in regard to students learning how to work together. I think on some level you’re right that they just have to learn by having more experience working on teams, but there are probably also things you can do to speed up that learning. I think that probably the best thing you can do is to encourage more meta-conversations about how well the teamwork is doing. Make it something they think about explicitly, reflect on and talk to each other about, and they’ll learn to do it better sooner. I don’t have any experience with group roles being explicitly assigned, but just having the students talk at the beginning of the project (and periodically throughout it?) about what they want their roles to be and what expectations they have for themselves and each other is probably a good way to start.

By the way, just so you know where this is all coming from: I’m currently a college student (engineering major, planning to become a teacher) at a school with tons of project-based learning. So, I guess all of this is from the perspective of lots of college projects, which have ranged in length from a few weeks to a full semester. Probably a lot of it will be less useful in your environment than it has been in mine, but hopefully it’s at least some helpful food for thought?