Honestly, you're a dental student to be! You should know that yellow teeth do not mean they're unhealthy! Also, whitening agents (if you get the cheap stuff) can actually wear down enamel, which is why dentists recommend to only use whitening agents provided by dentists themselves. If you're concerned, however, ask your hygienist for a quick polish before you go to Uni to remove any stains. I've found on my friends at least that it does wonders!