I feel the need to respond to this post considering all the negative responses.
For context: I lived in Ajijic until I was 6, then moved to the US. I recently moved back (I'm now 26) and reconnected with many friends I had here in Ajijic; They are now engineers, lawyers, on the way to become doctors, etc. Although it will be difficult for your daughter to adjust, there are PLENTY of amazing opportunities for her here and the private education is often better than in the US. GDL is known as the "Mexican Silicon Valley," in fact. Most of the kids her age will be bilingual by this point. I know a handful of teens here that speak English as a FIRST language because, as you know, this is largely a US/Canadian community and therefore, many parents/grandparents are from the North. But I really have to stress how amazing the education can be here. I feel confident enough to say that the friends I have here are better educated (often bi or tri lingual) and generally more successful than my classmates in the US (all from the same generation).
I saw many posts claiming that the youth here are involved in drugs/other unfortunate activities and I must say that's untrue. Of course, it does happen. But the kids that are involved in that often don't have any/many opportunities, parents, or are required to help their family with the bills. Typically, those kids drop out of school before they even learn to read/write in Spanish, therefore they don't have other great job opportunities and get caught up in what they think is "easy" money. It's a sad situation, but by no means are "all" the local kids associated in that nonsense. It's highly unlikely gringo kids, that don't speak Spanish, would be welcome into that world.
I also saw comments about Ajijic not being the place for "a Caucasian teen," which I feel is a pretty misinformed comment. Mexicans come in EVERY color, especially in Jalisco. This is due to all the European, Spanish, US, Canadian, etc. influence we have in the area. Your daughter will not stand out any more than any other teenager, especially in Ajijic because the locals know this area to be flooded with expats. More likely, people will assume she's Mexican first. Just teach her some street smarts and there's really nothing to worry about other than the language barrier/making friends immediately.
In fact, my mother and 15 y/o brother will be moving here this summer. We have similar concerns for him, it will be difficult and will push his boundaries but I think it's an INCREDIBLE opportunity, one I wish I had when I was a teen. To live in a country so rich in culture, learn a second language, etc. etc. etc., will be an experience like none other. Your daughter will come out stronger and with more life experience than most her age and I think that's priceless.
I hope this is a little more encouraging than the other posts, there are two sides to every coin! Being closer to your daughter's generation, I see things much differently and I'm excited for her! If anything, this isn't the place for 20-30 somethings. I'll vouch for that, haha!