Could might maybe.

There are probably a thousand alternative theories but you are wrong in claiming they have validity or should be taught in school. Maybe as an aid in a debate class but not as valid claims. Are you saying every alternative should be taught no matter how utterly ridiculous? If not then who chooses which are valid and which are not? Parents? A majority of parents? In this case it seems a minority of parents. No, it should be scientists and overwhelmingly they have chosen a single theory that global warming exists and we are the cause. They back this up via observations, temp measurements, core samples, etc. What you espouse is called "junk science" and we shouldn't be filling our childrens heads with junk.
If they'd only bring back prayer in schools all the worlds problems would go away. In fact we wouldn't need to teach anything other than praying your way out of a mess. Some out there would call that a valid argument. I call them crack pots.