It's a tough call depending on the circumstances and jobs. Very few jobs are truly once in a lifetime offers, particularly right out of school, so it is key to decide how much better it really will be. If one does leave Job A to take Job B, the other thing to consider is it positively commits them to the second job for a while, since a third company who sees two jobs in 4 months will be highly unlikely to hire the person.
If I thought i might find myself in that situation, I'd do as much as I could to find out Job B's decision before taking Job A, even to the point of calling Job B and saying that I had another offer I had to reply to, but preferred Job B and do they know when they might be making a decision. Since most candidates right out of school will be interviewing in multiple places, it doesn't seem like an unreasonable question in that type of situation. In addition, if Job B was so put off by the question that they wouldn't hire you for asking it, does that really suggest a place you'd want to work?
If I thought I might hear from Job B soon, I might call Job A and try to get a bit more time to make the decision, saying it's a major decision and I just would like another few days to think it over. In my experience, that is reasonable, provided the extension you are asking for is a few days, not weeks.
The potential for burning a bridge is really high in this type of situation, so while I think it is acceptable, I wouldn't do it. From an etiquette perspective, you can be polite about it either way, but it does seem bad form to leave a job that soon unless it is dramatically different than was represented. If the job was far different than represented, then while the risks are similar, I wouldn't feel committed to a company or manager to misrepresented a job they were hiring for.