Turn old school on (you need 2 people for, this 3 is better) go to the room that separates A and B.
Once you are there go to the back of the room and stop.
Next to the fridge there will be another table on the other side (hard part).
Get some one to get on the table and crouch jump on top of one of them and jump again to get on the ledge.
Once you are up you will see sheets of metal.
Jump on to one, keep on jumping across to the 3rd or 4th, then one step back and jump on to the building and explore!