http://z5lcip4dafatwwa6hvyibizpzwycvwp67cjga3hzjhxhwvuyaqavxnid.onion/math/497
They're not the most interesting problems I've ever seen, but given the huge quantity, it's pretty good. Unfortunately I don't remember them, but the most interesting problems from my schooling were probably from AMC or Titu Andreescu A nice problem I remember: Look at a map of something like state or city borders.