Hi Jen
I've so been there! Not wanting to "disturb" the teacher + felt like walking out so many times, and I've been in classes where I've been pushed really far. The only time I actually left was when I had a massive hangover & had to go and have a little puke
Remember, whenver you feel out of breath, or whenever you feel like the class is running away from you, that you can either go back to standing with your hands in prayer, or go back into baby pose.
One of the lessons I've learned over my time practicing, is that there is almost never any judgement from the teacher, and rarely judgement from your fellow pupils (we're all only human after all). What is best to keep in mind, is that you are following your path, and that Yoga isn't a competition class, its not about who can get deepest into postures, or who's Ujayyi breath or OM is the loudest. Try & focus on your body, and your breath, and if you've got questions, really do ask your teacher (or anyone who you feel comfortable with).
Its great your looking at practicing in your lunch, I wish I had the opportunity of a Yoga break in my dreary office day!
Reuben