Yeah, I definitely saw people with bags like that. My thinking was just that a student with a backpack would probably have been stopped - not for security so much as to check for alcohol - and I wonder if any gate guards recall him trying to enter and then leaving? With the bomb maybe on a timer? Just a thought...
There's a report somewhere on this thread, I think, that a bus driver saw him try to go in, but he left when they tried to search his pack. Then, according to the driver, he went to the bench, sat down, and started taking something out of the pack. If true, it would seem he changed his plans and decided to smuggle the bomb in on his person, and it went off as he tried to transfer it from the pack.