Following some posting on the Wilton forums, I solved my leaf problems by: 1, adding piping gel to my icing - luckily they had some at the NEX, I got the last tub! And 2, spreading open the tip with my spatula. Not perfect, of course, but a lot better. I'm getting there:
Of course now I'm even MORE annoyed with my instructor, because 1, she said the piping gel was unnecessary and not to bother with it; and 2, she inspected my tip and declared it to be sufficiently open - she said it didn't need to be spread any farther open. Sheesh.