Well, first of all, in your message,
1) The word (pronoun) "i" is always capitalized.
2) The word "because" not capitalized.
3) The first word of a sentence is always capitalized.
I would ask whether the test was also supposed to cover capitalization. For instance, if the teacher had also covered capitalization rules in the lesson, then I think she would be justified in marking off for capitalizing the 2 words you listed. The only exception is if it was the title of a book, magazine, story, essay, etc..
If you just want an argument to make, then you can claim that it is the first word of a sentence (although an incomplete one), and therefore it would require capitalization.
If you want to read up on it, here are some sites:
http://www.gpoaccess.gov/stylemanual/2000/chap03.pdf
http://www.angelfire.com/ks/teachme/capitalization.html
http://www.kimskorner4teachertalk.com/writing/conventions/capitalization.html
I recommend the first one.
Good Luck. I would also be upset if I were in your daughters situation. Of course I did take a professor to the Dean of my college once for giving me a 99 instead of a 100. I won.