Relax, kids. Please understand that you are never going to unleash your full potential as a performer if you do not chill out. So, firstly, begin to approach improv, and honestly any craft or challenge, with a vastly more lighthearted attitude. I often tell people that the only way to get better at improv is to fall in love with it. At its heart, improv is about exploiting the human train of thought for entertainment. You should not be afraid of your humanity when performing!
Take a look at a typical conversation you might have. Chances are you are going to stutter, forget what you were talking about, and, most importantly, say "um." It's not only normal to say "um" during an improv scene, it also doesn't make your audience think any less of you. Did you forget you're IMPROVISING?? NO ONE is out here trying to attack you for THINKING. The less brainpower you waste obsessing over that small "uh.." you let slip, the more think juice is going towards your improv scene.
I understand why we worry about these verbal imperfections. In the theatre, mumbling an "um" is typically associated with not knowing your lines, which is something losers do. Same thing with speeches. That is one way improv totally flips the script on live theatre: it doesn't ask for memorized perfection. All that improv wants from you is raw creativity. It doesn't want you to worry about having a seamless line of dialogue wherein you neglect the urge to take a chance because it may take you a second to say it right. It doesn't want you to avoid progressing the story in a kick-butt way because you MIGHT say "erm." But still, we associate those little "thinking interjections" in improv as a lack of wit or just poor improvisation skills. We think saying "um" EVER, even ONCE, is a sign that you aren't quick on your feet enough for improv. That is so wrong. The audience doesn't write you off as a crap improviser because you think. Stop being afraid of these verbal ticks popping up in your scenes. They say nothing about your skill as an improviser and you'll hurt yourself more worrying about them.
So, what did we learn today? Guys, chill out and stop worrying about saying "uhh." I've seen more kids completely derail the pace of a scene trying to avoid an "erm" than I have ever thought someone sucked because they said "uhm." Like I said, the less time you spend worrying about surface level stuff like that, the sooner you're going to see improvements in your skill as an improviser.