85 has a good line on extra letters of recommendation. Two additional things to think about when you are asking for extra letters of recommendation.
First, the Board knows the letter will say you are a great person, student, athlete, etc. They always state that. After all, you would never send them a letter that said something poorly about yourself. So, if you math teacher said in the first letter that you are a great student...sending another letter stating you are a REALLY great student may not have the sway you hope for.
Second, if you are submitting another letter, the person observing should have some insight into an aspect of your life they have not seen in your record yet and should have significant time with you to observe this aspect. A letter from the President saying you are a great person would have little impact since he probably has never spent time with you - unless it is Uncle Barack and he has seen you develop over many years. An additional letter from your math teacher would probably not have a huge sway either since the teacher still only sees you 2-3 hours a week in class and would again comment on how really great you are in those same 2-3 hours a week. So, if you are considering another letter, is there somebody in your life who could give additional insight on something the board has not seen about you?