Glasses have long been a stereotypical accessory of the "nerd" figure in stories. In the first 15 pages of text, I count 5 times when I can find Piggy cleaning his glasses. At each time that he is cleaning them, he seems to either be thinking about what he is going to say next, or dealing with the nervous apprehension he has in dealing with Ralph's teasing. It seems an almost obsessive habit, but for some people repeating a certain act helps them think.
Another perspective to consider when thinking about the glasses is that they reveal truths. For Piggy, when his glasses are clean and he places them squarely back on his nose, he can see the situation clearly. These glasses also reveal truths about Piggy. For example, when he gets sad and tears start to form the glasses fog up.
As far as intelligence goes, Piggy has it, and it is often revealed through an action with his glasses, but most people with glasses look scholarly.
Read the SYMBOLISM section of the attached link for more info.
If you are researching for a paper, look in the first 15 pages for quotes.