
Advice for this particular issue
A set of agreed team rules is always handy as it defines the boundaries. Remember that U16's are basically year 8 and 9's, usually midway through puberty. Year 8's have made the transition from primary school to high school and are seeking to assert themselves and push boundaries and show the world they are not some kid.
If you do bench for a breach of team rules, explain to the player why. I have seen too many dumb coaches bench a kid, not explain and when the kid asks "why? what did I do wrong?" the response is "well, when you have worked it out, you tell me"
A sense of belonging is important to most and the military do it well by instilling a sense of teamwork and being part of and contributing to something. Never underestimate the power of peer pressure to conform to a set of group rules.

I'd make a team captain that gets respected by all the team and get them to try and fire them up, try and get these underperformers to keep up with your captain and maybe push-ups as my only punishment but for the whole team to do!
I wouldn't bench them and destroy team morale and I would encourage the team when their performances lift ie. don't single them out but recognize everyone as a team and when the team work together then positivity should be enforced.
Good luck to your team

Aaah, the player(s) with the "lack of motivation"!
First of all, managing players is just like managing staff at work. If you don't have any (in) sub-ordinates at work, fear not, Dr Jack is here to help.
3 KEY PRINCIPLES
Self esteem is #1. If you tell someone they are useless and expect them to be motivated as a result, well choose another way to motivate people, particularly kids. I have seen coaches put kids down expecting them to think "I'll prove him wrong, I'll show him I am a good player!" Instead, it actually works the other way, players just think, "what a tosser". Therefore, "Maintain and enhance self esteem"
#2. Listen and respond with empathy. Communication is a two way street
#3. Ask for help in solving the problem.
When seeking to improve performance;
1. Describe the issue in a friendly way, what concerns you, and the standard you expect as a leader.
2. Ask for help in solving the problem.
3. Discuss causes for why the issue arose.
4. Identify and write down possible solutions.
5. Decide on a specific action to be taken.
6. Agree on a followup date and time.
It's a pretty simple formula, and you'll find it will work.
