Is it ok to end a sentence with the word had?  I used the following sentence in an informal reflection: I wonder if the principal had as much support as he believed he had.

My professor down graded the assignment because she said it was inappropriate for a graduate student.  Is this so?  I taught English for years.  I never once discussed this rule with students.  Please advise.

I'd say it depends on how informal 'informal' means.  Your professor takes a stricter view of 'informal' than I would. You'd better be more careful next time!

Also, in informal reflections, can a bulleted list be used to summarize major points of an article?

Again, it depends on how informal you are allowed to be. I certainly would advise you not to try that with the same professor. I wouldn't much approve of it myself, either.

Best wishes, Clive