I suggest you buy them from the pharmacy or pharmacy section in your grocery store. Those folks are used to talking about all sorts of weird health problems, they aren't going to bat an eye at someone buying a box of condoms.
I know your feel bro, when I was in high school I was a bit nervous to do it too, but once you get older you'll realize it's no big deal. Just remember that there are probably hundreds(or dozens?) of people buying condoms there every day. Like the others said, walk in with confidence and you'll be much happier with yourself
