NEW YORK — Denim reigns on several Northeast college campuses, but it’s more spruced up this fall. At Columbia, NYU and Harvard, fitted jeans and blazers have replaced the baggy jeans and oversized flannel shirts of the recent past. It’s not exactly preppy, but the look is classic and clean. Other popular looks: tunic sweaters and leather jackets with jeans. One emerging trend is the large side shoulderbag for toting books in lieu of the popular backpack.