Well, overall its up to you to decide if or if not you want to disclose that information. Regardless of what you choose, there will be positives and negatives on both sides. If you mention that you have ADD, you should be granted access to things that could really help you do better in class. Like perhaps extra assistance before/after school and longer time to work on tests and such. I've notified some of my professors about my ADHD (severe lack of focus and I get distracted easily). Anyway, they were very understanding about everything and gave me a better environment/extended time for tests. It helped me a lot. On the flip side, not all teachers will be that understanding about everything. In most cases however, I would assume the teacher would be supportive on the issue. I really don't see any harm in mentioning it though.
Best of luck to you in your studies!