I would first find out how the school knew he visited the youtube page for a pen bomb. Assuming it is a browser history I would ask to see it. Were there other inappropriate sites or videos visited?
I would ask for a copy of the schools internet use policy.
I would ask what controls the school uses to prevent inappropriate websites from being accessed.
I would point out that the "pen bomb video" is not really an instruction for a bomb at all. It's a stupid party favor modification.
I would also point out that even if youtube's "safe" mode is toggled on this video still shows up.
None of this adds up to a 10 day suspension. Usually I come down pretty hard on kids who misbehave at school and side with the administration but this is over the top.
If your son is suspended for 10 days ask what discipline was taken against the IT person for not blocking youtube videos, or against the teacher for giving an assignment that, when you consider it, was pretty poorly worded. "how to build a fire without a lighter or matches" is bound to have search results that are questionable. Did the teacher give instructions on what resources could be used? Did the teacher do a search prior to the assignment and see what kind of results would show up?
Or was this just the last straw with a kid who has been walking the line for a long time.