At my high school we were taught to use Word and Excel because those were considered essential skills. We could have learnt Open Office or something else but really, how often are we going to encounter that in the real world? In terms of practical use, .Net is not a bad choice.
Personally I would like to see Python for getting started and practical use and maybe Scheme for going deeper.
I'd say let's take practical-business-relevance out of it (even though I'd argue the relevance of Java, Python, and Ruby to be plenty comparable to .NET).
We're talking about high school students, not trade-school students. I think the curriculum ought to be about opening their eyes to this really incredible world of solving problems with computers and logic. I also think a bit part of the key to that is to make sure we don't saddle them with skills that can't afford to develop. You know how bummed I was after taking AutoCad or using Photoshop in high school only to find out that I'd have to mow lawns until I was thirty to afford to use them at home? That's just stupid, and spirit-breaking for interested kids that don't have 800$ (or whatever) for Visual-Studio-Enterprise-Resume-Edition.
I think the best argument here is regard students running Macs at home which I have no reasonable counter to.