One main concern raised by both groups was this--how can you be sure that students are learning all of the content that you need them to learn using this method? What I have learned about PBL and content is this--while you can plan for students to discover the content you want them to learn when crafting the original problem, sometimes students take the problem in ways you never expected. And you should let students take it there, as long as they are engaged and are learning--because PBL is really more about teaching students how to learn much more than teaching students content, in my opinion. Also, if you plan on using PBL as a consistent method of instruction, you're going to have to sacrifice some of the "nice-to-know" parts of your content, and focus mainly on what you want students to walk out of your room remembering forever and ever--the "need to knows." I think teachers have to start thinking about instruction and content differently when using PBL--they have to let the learning determine the content they teach, rather than the content determining the lesson plan for the day. In other words, they have to first determine what they really want students to learn (problem-solving, critical thinking skills), and then choose content that will help students do that learning.
Another great point that was brought up was this--what if students are discovering and learning the content on their own, but they are inaccurate in their understanding? The key here is catching these misunderstandings early, and then taking steps to correct them. In PBL, you can't always leave students on their own to do research and generate solutions to the problem--formative assessment at the end of each period (and walking around listening to their learning conversations during the period) is a critical step. I administered progress checks quite often on the I can statements (objectives) students were responsible for learning, and if any misunderstandings reared their ugly heads, then I planned some remediation strategies for the next day. For example, when I was doing my PBL genetics unit, I had some interesting yet inaccurate interpretations about how traits were inherited after students had gone through the online activities designed to teach them the basics of heredity. Because I had given them a progress check after they did the activities, I realized where students were going wrong, and then designed some stations for them to do to get back on the right path. In PBL, formative assessment is a critical step to check and see if students have an accurate understanding of the science or math concepts before they apply those concepts in generating a solution to the problem.
The biggest concern that was raised was the fact that students are not "trained" to learn in the way that PBL calls for--for the learner to do the work of learning. One of the main reasons I like PBL is that it places the "burden of proof" for learning on the student; however, this is the main reason why some students don't like PBL, and why some students will experience frustration with learning to learn on their own. This frustration comes from the fact that PBL doesn't smooth out the learning bumps in the road for them. PBL allows students to set off on wrong paths, make mistakes, and re-route themselves in another direction. It also allows students to experience the real-life phenomenon of not being able to find the "right" answer--because if the problem is set up in an ill-structured, messy manner, then there won't be just one right answer. For students that are used to being able to look up answers in the back of a text or on the internet, PBL can be a very uncomfortable experience with the cognitive dissonance reaching raging levels at times. So, how do you deal with this student frustration? I try and give students a heads-up about the frustration by telling them before the unit begins that, yes, they might get irritated or annoyed when I just ask them questions rather than giving them the answer, and that they may get frustrated when they find out the research they've been doing is going in the wrong direction. I also tell them that it's OK--and if they start feeling as if they are going to have an outburst (which has happened), they should come to me, and we will see if we can ease that frustration. In other words, I try to give them a safe place to go when it all gets too much for them, when it starts going well beyond the norms of the factory-model system within which they are used to operating. Really, it's about telling students how PBL is going to be a different experience for them up-front, and giving them somewhere to go when frustration arises. Interestingly enough, the students that are prone to experiencing these frustrations with PBL are the students whom we traditionally label as "high achieving" or the "smart kids" in our room.
As you can see, there were some fantastic points raised about implementing PBL in the classroom during my sessions, and I think these are issues that every teacher who wants to try PBL should be thinking about ahead of time. You can check out my presentation below, and if you have any other issues or comments about putting PBL into practice in classrooms, please feel free to share.