Thanks for the additional information.
First, a commercial burglary is much less serious than a residential burglary. So that's good.
Second, you mention the prior record of the others. This is a valid point that should be taken into consideration and is discussed more below. However, you mention that they "robbed" the store before, but I wonder if what you mean is that stole something from there before. While it is very common for people to describe a theft as a robbery, most thefts are not robberies under the law. A robbery occurs when the thief takes something from another person or their presence using fear or force against the person. Stealing something from a business when no one is looking (e.g., shoplifting) and/or present (e.g., commercial burglary) is far less serious because no person is placed in danger. Also, a commercial burglary is less serious than a residential burglary.
So back to your main question - yes, you're correct that the prior criminal history of the others should be a factor in determining the appropriate sentence. However, there are many other factors that come into play, such as the role of each defendant, their relative ages, the degree of cooperation of each, the extent and sophistication of the planning involved, the anticipated loss if the crime was completed, the statements provided by other defendants describing your son's role, and so on.
It sounds like your son needs to sit down with his lawyer and find out why the lawyer is unable to use your son's lack of record, his performance at school, and his future academic plans to secure a better deal. All of those factors are pertinent to the negotiations. Either the lawyer has already made this pitch - in which case he'll be able to explain the prosecutor's response - or he hasn't. If he hasn't made the appropriate pitch, you and your son need to get have a serious talk with the lawyer to make sure that he makes the attempt.
You don't say whether your son is charged with a misdemeanor or felony. This crime is a wobbler - can be charged as either felony or misdemeanor. So your son's lawyer could try to get the judge to reduce the crime to a misdemeanor. The judge can do this without the prosecutor's permission.
Another option is for the lawyers to informally discuss the facts of the case with the judge and see if the judge will try to encourage the prosecutor to cut your son some slack. This really depends on the practice and personalities of the players involved. Some judges stay out of negotiations entirely and simply will not give any indication of how they view the case.
And, yet another option is for your son's lawyer to negotiate "a lid" and then make the pitch to the judge after the plea. A lid is when the parties agree that the defendant will plead guilty with the understanding the maximum the court will sentence him to is X (whatever the prosecutor was looking to get). X is always far less than the statutory maximum. After the plea, each side would have an opportunity to explain why the court should sentence according to their recommendation. Let's say the prosecutor was looking for 60 days and the defense wanted straight probation (no jail time); each would argue their case presenting mitigating and exculpatory factors to the judge. The court would rule and sentence somewhere in that range. The benefit of this kind of deal is that the prosecutor gets his guilty plea and the defense has a shot at convincing the court to cut your son some slack.
Unfortunately, the lawyer can't force the prosecutor give him the deal he wants; all he can do is try to persuade him using the mitigating factors in this case. If your son and you are not satisfied with the lawyer's efforts in this regard, you can hire a new lawyer. If the prosecutor is set on not reducing the offer, a new lawyer may not fare any better.
I hope this is helpful. After you click Accept, I would be happy to answer any follow up questions you might have.
Good luck - Deirdre