What should I wear to a semi-formal event?

I am a male 18 year old. My friend is having an 18th birthday cotillion, and for the dress code, we have to wear something semi-formal. What should I wear? I'm planning on wearing dress shoes, black slacks, white or black collared shirt, and a tie. Is that too formal or is it good enough? I really don't want to show up overdressed or underdressed.