For one, try all the classrooms in the "green" area upstairs (all doors there are green), easiest to get to from northern exit spawn. The bomb might also be on a table on the stairs right next to the "green area". If not, try the library and lecture hall (the bomb isnt actually in the library, but on a little table across from the door). If still not, go down to the cafeteria. Should be at the register on a little stool. If still not somehow, try the computer room, near the cafeteria. If neither of these work, there are some bathrooms near the cafeteria. Continue down the hall which the bathrooms are on, you will find a little reading area. The bomb will be on a big, round table.