Especially with younger students it does not take long before they have no idea what notes they are actually playing, and are just listening and watching me and then copying. I usually extend their range up as high as possible without excessive tension or pressure. I will then ask them to read some music, and I quite frequently get the response "I can't play that high" or something similar, even though we just played considerably higher with no problem (though they don't know that). I tell them to play it anyway, and most of the time they do fail in their attempt to hit that high note that they were convinced couldn't be hit. I then show them that they actually just played higher, and we go through part of the warmup again with them knowing this, but still just focused on the sound. Now when they attempt the impossible piece, they can magically play that high note.
Except of course that it's not magic. As my previous posts have been pointing out, thinking about the possibility of failure invariably leads to the kind of tension that causes problems on trumpet (and virtually anything else you do). This is one of the reasons that good practice is so crucial. Practicing in a focused, consistent manner leads to consistency in playing, which leads to less thoughts of failure, which leads to better playing. Like I said in my last post, Clarke studies are great for this (with a metronome), focusing on sound, precision, and airflow (Simply focusing on sound will take care of precision by itself, but you must listen very closely and have a clear idea of what you want to sound like. You get ragged edges in your playing when you don't have this clear idea).