I would say they were in three main areas:
1. On the computers
2. Sitting on the floor in the fiction section reading
3. A group of late tweens who seemed to be friends browsing at the end of non-fiction 500's. I looked that up and the upper 500's is biology/zoology/animals
The others were just scattered about browsing or sitting at tables reading. The really younger ones were with their mother (presumably) at a table looking at books.
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The library does have a bike rack out front. There were several bikes parked in it so one can presume that some of the boys biked over. That is especially unusual these days as it is rare that kids are allowed out on their own. Of course, they may have traveled in groups which is more acceptable.
Boy in jeans had a nerdy look to him. He was in jeans, a white t-shirt with a NASA graphic, black sports sandals, and wore glasses. He had sort of messy blonde curly hair that was faded around his ears. He was at one of the computers very intently involved in whatever was on the screen.
Are nurds, no doubt unsporty, shyer about their bodies so even in hot weather choose to be covered up. Maybe there were one or two boys late to tackle school holiday-time assignments. Girls would naturally all have them done already being the more sensible creatures they (usually) are