OK, may just be I don't have the patience right now to do this, but ... how do you ice cupcakes and not get crumbs all over the icing? Do they need levelled first or something that I'm missing? I have my Wilton class tomorrow night, and need to bring them there ... and at this point I'm starting to not care what they look like, just to get them done!
Make sure that your icing is a thin consistency. Can you use a 1M tip and make the swirl on them?
I always pipe my icing onto the cupcake in a swirl with tip 1M, it make for much easier/faster frosting, i just set them all in the container they will be carried in, or the plate i will be serving them on all pipe the icing, and it takes me 5 minutes to do. Plus it looks very professional! Hope this helps. 
Love to do the swirl (maybe I should say love to try to do the swirl ) but since this is for the class, think it has to be smoothed out, so we can add decorations/figures/something -- I don't know. If I manage to make it to class tonight, guess I'll find out!
If there's even the slightest hump on the top of the cupcakes, that's why you're getting crumbs in the icing. I've had that happen before, too, and I know how frustrating it is. Maybe try leveling the cupcakes by cutting off the hump. That should make it easier for you to keep the crumbs out of the icing. Good luck!
When I need a flat icing on my cupcakes, I pipe the icing on in a circular pattern with a #10 or 12 tip, then use a small spatula to smooth it out (to get a really smooth surface, I dip the spatula in hot water, wipe off, then smooth the icing). Piping the icing on, then smoothing helps reduce picking up crumbs form the cupcakes.
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