Bryan, you should change the title of this thread to "What if the bombs had gone off?" That way people will know what the thread is about.
It is often said that if the bombs had gone off Columbine would have been "much worse." I believe this to be untrue. In fact, I think fewer students would have died.
The first thing you need to understand is that Eric and Dylan used a BLEVE device, which means that the bombs would've exploded in two steps, the first a small explosion that would have started a fire, and the second a much larger explosion brought about by that fire heating a propane tank, as you can see here:
[You must be registered and logged in to see this link.]If the bombs had gone off, the students would have witnessed a fireball, not an explosion. And in fact, Dylan successfully detonates one at 1:10 here, so you can see what the students would've seen if the bombs had gone off--a fireball which quickly dwindles into a very small fire.
[You must be registered and logged in to see this link.]At this point, the students, almost all of whom would be uninjured, would have fled the cafeteria. They would have fled out of the series of six southwest doors and the northwest door, while others would've run upstairs through the north stairs.
[You must be registered and logged in to see this link.]Those fleeing out the southwest doors would've probably encountered Harris fairly soon, though. And he would've begun shooting. But because there are so many doors (and the fire would've been small), they all would've run right back into the cafeteria at the first shots.
Those fleeing through the northwest door would've encountered Dylan, but who knows what he would've done. He barely fired in the beginning as it was. In any case, this was a single file door, and the few that got out would have quickly turned to run back inside and up the north stairs.
At this point, Eric and Dylan would have been standing outside right as Neil Gardner drove up and they would not have been together. Both would've been exposed to fire (in fact, Eric would've had his back to Gardner) and we know that Gardner would not have been hesitant to shoot them. Moreover, Eric and Dylan would be separated and unable to coordinate their actions.
Another possibility is that Eric and Dylan would've pursued the students into the cafeteria. This might have led to a higher casualty count, especially if they had gone upstairs, where the students would at this point have been following fire drill protocol, but I don't believe they would've done it, because Eric was extremely scared of his own bomb. In fact, at the slightest sign that Dylan is about to detonate it Eric takes off running at 1:22 in the above video. They would've stayed outside and it is my opinion that Eric would've been dropped by Gardner. At this point, Dylan would've panicked.