Damaging Distractions

My cooperating teacher runs two faster-paced enriched courses for his 8th graders, and for their class we're giving them a group project where each team of 4-5 students has to teach their classmates about a topic they haven't gone into detail on yet. Students picked topics like electrical circuits, sound, light, and electromagnetism, and have to teach their peers the key points, show them a demonstration, and give them a quick assessment on what they should have learned.
Many of the students wanted to see their phenomena in action before just reading the chapters cold, and rather than wasting tons of time retrieving equipment and setting up labs that might not work well, I decided to encourage them to use some online lab simulations. The teacher thought this was a great idea as well, and we showed them a site that had some great programs that would quickly take the place of the more time consuming and labor intensive labs, and really give them great insight into many of the fundamental ideas behind their topics that they could start with.
This was going great, until the students devolved from learners into browsers. Within moments of showing them the sites they could get the labs from, most students had opened up multiple tabs in their browsers and were pulling up songs on YouTube and a multitude of other focus-draining sites on the side. It seems that the firewall on the network does block out obviously noneducational sites like Facebook and Twitter, but there is just no shortage of procrastination enablers on the web, and our 8th graders wasted no time in figuring out which ones they could get away with pulling up.
It's hard to assign blame to them, as we as adults are (generally) not much better. By now, most of us have made the connection that when it's crunch time, we can only truly get work done by being disciplined about limiting what windows we keep open, but it's undeniable that using internet resources to be productive comes coupled with the inherent temptation to fool yourself into opening up the forty things you 'need' to keep checking on.
For our students who haven't gotten to this point, how do we illustrate the danger of letting distractions seep in? In some cases we can try to catch them and reprimand them, but if they're really expected to be able to take advantage of these awesome tools, we need to be developing their mindsets so that they become aware of their tendencies. At what point would they be mature enough to take ownership of something like that? Is specific education on how to wisely approach browsing something that needs to be implemented?