Do you think it is too late to send an update about an achievement? I recently found out that I qualified for a math competition after being in the top 700 students of 45,000 participating. Is this worth it?
Thanks for the feedback! 
It likely won’t make a difference in your decision, but it can’t hurt. Just be sure to make the note brief and to the point as admissions people are busy this time of year. If any school is your top choice, you can use the note to reiterate that as well (ex. “if accepted I will attend”)