Best Prime Lenses for Nikon?

Hi,

I'm a paid photographer at my school, and I'm looking to upgrade/change some equipment. Currently I have a Nikon D3100 with two lenses: a 15-55 f/4.5-5.6 kit lens and a 70-300 f/4.5-5.6 (guessing on the aperture there). I use the 70-300 for shooting sports events and other outdoor events at school. I'd love to get more into portrait photography, and I was wondering if I should get a 50 or 55mm prime lens or an 85mm prime. Or should is that silly with such a baseline camera body? Should that be the first thing I upgrade? Please advise.

Thanks :-)