In my current job I work with, almost exclusively, high school-aged athletes. The kids are either currently in high school or in the prep year between high school and college. This, obviously, creates some interesting situations in the weight room.
High school kids...especially boys...are idiots. They are more interested in talking shit to their teammates and trying to make people laugh then they are in listening to directions. Getting results with a group of athletes this age is really pretty easy; they have a very low training age, and as long as your programming doesn't really really suck then you're going to see improvements.
The issue is getting these guys involved (mentally) enough into the workout to do what they need to do (especially in a group setting). Even the most dedicated athletes at this age have a tough time focusing, especially with some of their external stressors. Applying to colleges, school work, relationships, etc. Everything that happens is the end of the damned world and it affects their performance.
One of the biggest things to contend with in this environment is the bravado that comes with lifting weights. Athletes this age don't typically know anything about exercise prescription or practices, they just wan't to get yoked. They will typically regurgitate awful information that their brothers friends uncle told them, and he (obviously) benches 600. Even when applying percentage based programming to these athletes, they will have the tendency to continue to push the weights with little regard for form or tempo. The coach programmed paused bench press with a 32X0 tempo? #YOLO I'm just going to load up an extra 20kg and bounce this shit off of my sternum a bunch of times.
You have to be pretty diligent about riding athletes this age (really, any age) about being strict with their tempo and being honest about the way the weight feels for a unique set or rep. Damn near every time I approach an athlete and tell them to decrease the weight for the next set, I get a look back like I just told them that they were adopted and that their mom was really the Tooth Fairy. No matter how often you tell them that it is not a personal attack on their masculinity or ability, they take it as such and usually demand that they get another chance at "making it look better".
The key to avoiding this situation is letting the athletes know from Day 1 that being in the weight room is not the proverbial dick measuring contest. It shouldn't matter to them what anyone else has on the bar, and nor should it matter to anyone else what they have on the bar. Their job isn't to become an amazing weightlifter or powerlifter, but to get better at their particular sport. I also don't like setting anecdotal limits like "a girl can lift this" because 1) that's demeaning to women and 2) There are some really fucking strong girls out there who out-lift a lot of people. The particular athlete you are aiming these anecdotes at is probably not choosing to be at his current strength level and since he's in the gym with you, you have actual physical proof that he is actively trying to get stronger. Telling athletes that they have to get stronger because so-and-so lifts more than them will result in sloppy movement patterns and poor tempo executions.
One of the other big things to think about when dealing with a group of teenaged athletes is their extreme propensity for bullshit. Just the other day, I was warming up a group of 18 year old basketball players. When instructed to perform inchworms I was received with a round of grumbles and a few guys who asked if they could skip it. I inquired if they really expected me to say "oh sure, just don't do it because you don't like it" and they knew that there was no chance I would let them skip it. I made the simple suggestion to them that they just stfu and do the work; if they spent as much time doing the work they should be doing as they do complaining about it, they would be good at the exercises and they would no longer suck as much.
Keeping your athletes involved and understanding why doing each exercise/drill will help them get where they want to go is amazingly important, as is helping them to realize that they occasionally just need to shut their mouths and do what the coach says for the simple fact that they are the coach.
Be firm, be clear and encourage your young athletes if you want to help them get the most of their training and prepare them for further training down the line. Really, you don't want a poorly trained athlete moving on to college and saying "hey, I learned to lift with So and So" and having their new coaches be like "well that person must suck, because this athlete lifts like doo doo".
Have a great day, and go lift some heavy shit!