I definitely consider ASL to be a foreign language. I'm teaching my third daughter and also tutored another homeschool girl all last year. The area college has several ASL classes, as well as an interpreter's certification program. It is a bona fide language with a grammar all its own that has grown out of a distinct people group with a culture all their own.
You're right, though, some colleges recognize it and some don't. So, unfortunately, that might have to be a deciding factor; but if a child is interested in ASL, let him pursue it along with another language, if needed.
I'm afraid I don't have a lot of good resources for studying ASL. I learned it the living way from my mom, who was an interpreter, and her deaf friends. So when I teach it, I just teach it by doing. :-) One Web site that has been very helpful is ASL Pro. They have great video dictionaries and fingerspelling practice.
The very best way to learn ASL, as with any language, is to spend regular time with those who speak that language as their heart language. The deaf community is usually pretty tight-knit; if you can find a deaf person, he or she will most likely know other deaf in the community and any events where you could interact and practice your signing. For example, here near Atlanta there is a monthly game night for the deaf community, as well as a monthly "Silent Supper" held at the local mall. These are gathering places for the deaf, and they usually welcome ASL students too. You might Google "deaf" and your city and state and see what organizations or events you can find. Or you might check with any local churches who interpret their services and see if you can make some connections with some deaf people.
Hope this helps!