I bought generic supplies -- mechanical pencils, pens, paper, highlighters, folders and binders because that's how my kids prefer to organize themselves for classes. They're both in HS and we'll find out at the open house if their teachers want anything specific beyond what I've already got (not likely). DD1 needed a new backpack, so we ordered that.
My standard BTS shopping is: One new outfit for the first day, new athletic shoes, and a pair of fun/casual shoes that they can alternate with their sneakers. Plus replacing worn out/outgrown underthings. DD2 got new shoes over the summer for XC training so her new athletic shoes are already taken care of, but I'll probably get her a pair of spikes for meets.