Q:

What's a realistic distance to expect boys aged 11-17 to sea kayak in a single day? 

I'm thinking about taking my Scout troop (boys aged 11-17) on a sea kayaking trip.   What expectations should I have for the distance that we can travel in one day?   Assume good weather (clear skies, very light winds, and calm seas). 

A:

If you don't know what your boys are capable of then it's unfair to have expectations. 
I'm a Venturer Scout adviser for Scouts Canada; I take boys in that age group on trips regularly.  I don't know what your group is like, but if it's anything like mine, then the boys probably all have different levels of ability, strength, and skill.  For trips like this one, you can only go as far or as fast as the slowest boy.  My experience has taught me that you can't realistically set goals for distance unless you already know what your boys are capable of.  
Before you even plan your trip, I would recommend that you plan a day or two of kayaking just to see what your boys in fact can do.  Don't plan on covering a certain distance, just plan on being on the water for a certain amount of time.  You can then use this time to answer your own question by keeping track of their progress over a certain amount of time or distance.  It's important make sure that your boys also have a firm grasp of basic skills, and how to respond in case of an emergency.  I always take my boys to a local lake just to have them paddle around together before I'll consider taking them on trips.  I give them all a quick instruction, then let them play for a bit while I observe so I can get a sense of what their natural ability levels are.  I'll get them to play games too; races are a good way to separate the fastest from the slowest.  If you're using two-person kayaks, this can help you balance the group by pairing the stronger paddlers with the weaker ones. 
If you know you won't have an opportunity to assess your boys before you take them on a trip, then you need to either plan a trip that anyone of any skill level could accomplish, have more than one plan in case Plan A doesn't pan out, or supplement the trip with other activities.  South California is obviously different than up here, so you probably can't just tell the more adventurous boys to paddle off and explore a nearby island while the exhausted boys hang out by the fire and rest their aching arms, but you could perhaps have them paddle to the horizon and back, or have them create an obstacle course where they paddle to a point and back, then sprint-portage the kayak down the beach, and paddle to another point, etc...
I gave up having expectations without experience a long time ago.  Get to know your boys, have them set goals, then let them have the expectations. 

A:

Most grown people are good for an hour or two of constant paddling.  Children get bored more quickly then they tire.  People who go farther are either more athletic or more disciplined.  I go fishing in my kayak, and my fishing grounds are about an hour's paddle each way (depending on currents and wind), and I spend an hour or two trading between drift fishing and paddling back up current again. 
 At the end of that, I am pretty worn out, my butt hurts and I don't feel like doing anything at all physical after that.  Sometimes catching a fish is a curse when you get back and have to bend over and clean your fish. 
In addition to what Shen wrote, first drill then in self rescue, spend a whole afternoon on it until the boys can slip back into their kayaks like otters.  This is perhaps the most important excersize/drill you will do before bringing anyone out on the water deeper than they are tall.  I drill with my kids the beginning of EVERY summer before we go paddling anywhere.  Self rescue, swamping kayak or canoe, any of the rescue type scenarios including pretending you got knocked out so they can rescue you.  Thankfully my kids think it's fun. 

A:

On a sea kayak you might get significantly better results but my 9yr old could maybe do a mile on our rec boats.