We use padlet.com (formerly, Wallwisher) a lot in my room. We have used it to show our learning at the end of a unit of study, we used it for the Global Read Aloud this past year and invited other classes to comment on the book Charlotte's Web, and we have created a wonder wall to write down our wonders during the day.
I explained what we were doing to my students, they got their iPads and partnered up with another student (We have 20 iPads and I team teach with another teacher, so there are 40 students) and they scanned the QR code for the What I KNOW wall. I would suggest having more than one QR code to scan because I had 2 and it took a couple of minutes for everyone to scan it.
We then moved onto the W part of the chart. Students again went up and scanned the QR code and started typing what they wanted to learn about frogs. On padlet you have 2 choices on the layout. Free form where you can place your comment or question anywhere you want or stream where the posts are placed one below the other. We tried the stream for the second wall but because the stream kept moving we had to change it to free form because my students kept losing their spot when they began to type.