The only reason I think you shouldn't learn subs is that when switching to functions I found it hard to remember to put the braclets and remember to put the arguments separated by a comma, etc. I was saying that since functions are so similar you should teach them instead to avoid having to partly relearn something when moving to harder languages.
Isaac Waller
http://www.isaacwaller.com/
I am 12 years old and now program Java and C++, but when I started I programmed FreeBasic (basically QBASIC.) It was nice and easy and FreeBasic has many advanced features as well so you can learn at any level. But 1 thing - don't teach SUBs - teach functions. Believe me, it's really hard to move from SUBs.
Isaac Waller
http://www.isaacwaller.com/