nerves, mental errors

All you can do is all you can do - but make sure you take care of all the essentials (preparing them physically and mentally for the situations they will face) and then next would be to determine what extent they are anxious about.

We've had kids that would have (legitimate) panic attacks (see that thread) during games
Have had a running back dying to get in, then when the starter was out and this kid was poised to START his first game.....he ended up in the ER with anxiety attacks / vomitting / diahhrea (it was all due to how he worked the fear up in his mind about starting this game).

Boyd Epley had a pretty comprehensive methodology back in the late 90s incorporating mental imagery and physical stimulation (breathing,relaxation, controlling your blood pressure)

1. Part of the issue, may be EMOTION. Competing should really never be about emotion. Emotion can work you into a frenzy and never know why you are doing the things you are doing. Go into the game with controlled tempo and approach it like you're just going to have another practice (execute, execute....go to work). IMO, this is a big part of HS, as teenage kids don't know any better. Do what you can to DIFFUSE the emotional triggers (some kids work themselves up to be angry, etc) to prevent mental fixation that create lapses.

2. Part of it may be insecurity.
If kids can perform dead-on in practice, that is one thing (can do the assignment).

If the kids can perform dead-on in practice versus the looks you will get Friday night, that is another thing (can do it against game situation / simulation). Its not enough to know to be able to throw the post, but to know to throw the flat instead, when the curl defender gets depth (takes away the post).

If the kids know where their security / safet net is (what is, what isn't important), they will be less prone to {censored} their pants. Take for example a quarterback that knows that when the stuff hits the fan, he can always go to his outlet (flare/shoot) receiver (he can never be presented with an "OH SH** situation).

If its just one kid, develop a routine for him to give him comfort/security. Something to condition him / work himself into a peak physical / emotional situation (think Tourette's)
something like;
1. Play is over.....
2. Get to huddle (rally point)
3. Take 3 deep breathes using a 7:3 count inhale/exhale ratio
4. Listen for the call
5. What is the formation?
6. What is the cadence?
7. What is the play?
8. What is my angle/route/assignment?
9. Breathe
10. Repeat 5-8
11. Align
12. Deep Breathe

Initially, you may need the player to verbally call each step out and rehearse it. It is important to go through each step and review it with him when he comes back to the sideline (quiz him by going through 5-8 with him....). This is compartmentalizing the focal points of his performance. This is like being able to go up to a DB after a particular play and get immediate feedback by asking if he was in-phase or out-of-phase with a receiver....there are KPIs to meet (and thats all you have to worry about).

If you wiped the floor 70-35, sounds like you had an inferior opponent, so would these exhibited 'nerves' be present only when facing legitimate competition? If so, what are they doing to stay focused and not be intimidated?

In any event, keep doing what you're doing. It may be frustrating this season, but your method for building the program works and is set up for long-term success. Just be encouraged that your pride points, emphasis on participation/buy in, and dedication is the way to go and there are no short cuts. Good luck