If you're going to have to wear a retainer everyday for something like work or school, buy a better retainer than you can get at the mall. Retainers from places like Hot Topic are for temporary wear and will work in a pinch, but for something you're going to have in your piercing every day get something that will work better.
If you speak to your piercer they should be able to find you a quality retainer made of PTFE or even quartz glass. You will want a retainer that is styled to stay in place. The retainers at Hot Topic are usually just little sticks of plastic and they love to fall out. So look for a retainer that is shaped like an L or one that has a little corkscrew. If those bother you, you may be able to find a nosebone/pin style with the little bead on the end of a straight stud. The point is to get a quality material to keep your piercing healthy and a design that is comfy and stays put. If your piercer doesn't have what you need, you can find nostril retainers of plenty of websites.
When you wear a retainer every day like you're going to need to, leave the retainer in place unless you need to change it. For example if you're just going home after school to sit around the house, don't worry about switching your jewlery to another stud. Leave the retainer in. If you're going out with friends you can always switch to a prettier stud, but try not to change your jewlery every single day. Switching jewlery too often can aggravate your piercing, so if you don't need to show it off leave the retainer in.
If you purchase a good retainer and wear it correctly, you won't need to cover it with junk like concealer. A good retainer will keep your piercing healthy and get you by at school.