If it's a "zero tolerance" policy, and he violated it, or the policy/rulebook states that any act of physical aggression is punishable by a suspension, he unfortunately doesn't have any rights to claim that he shouldn't receive that sentence
just because someone else did something worse. The school also isn't legally required to consider how a person's college prospects could be affected by the punishment. In this scenario, if the board did consider that, they very likely would get accused of favoritism.
Look at it in a criminal
context: If the punishment for battery is 3-5 years, and I punch someone in the face, I could get three years. Another person who beats someone badly could also potentially get three years. As long as the sentencing guidelines are followed and were not designed to punish people of a particular race, there is no ability to claim that a person either had a right to a lesser punishment or that the other person should have been punished more harshly (and there is no way to go back and make that happen). It's the same idea here. The school has the ability to punish someone, consistent with the rules and the punishment guidelines that are established.
The board would have the ability to review policy to see if there was a way to amend it, consistent with the state guidelines. If the board has the ability to overturn a person's suspension, that could be within their discretion as board members. I am only explaining why the original punishment is likely permissible, and why the board would not be required to change it.