Will top 15% ranking ruin application?

My ACT is 34 and ranking is 15%. EC, essys, and LOR are strong though. Do you think the ranking is a problem for me?

BTW, I am from a very competitive public school.

Absolutely it will not. Top 15% is still very strong, especially at a large, competitive high school. Your ACT score is also easily within Northwestern range (especially if you applied ED). Top 15% isn’t perfect, but it’s not enough to “ruin your application”.