The lawsuit isn't frivolous. The lawsuit isn't to prevent his son from getting punished. The lawsuit is because the school apparently didn't follow due process. I.e., it handed out a punishment in excess of what it published would be the punishment.
It's not a matter of the father trying to get his kid out of trouble - the father agrees that the kid broke the rules. What some of you are arguing is similar to "well, he was going 5 miles an hour over the speed limit, so they fined him $100,000. I can't believe that the father is saying that it's okay to speed." He's not - he's saying that the punishment wasn't the correct punishment according to the school rules. If the school has a written policy, then they MUST follow that written policy. In the case that there are two policies, it should be up to the court to determine which supercedes the other.
Yes, the kid signed a code of conduct - but there are apparently two separate sets of rules at the school - one says it's after the 2nd offense, and one says it's after the 1st offense.
Should the kid be punished? Absolutely. Do I think the rule should be that the kid is thrown out of the honors class? Absolutely. However, the school can't change the rules arbitrarily or have two sets of rules for the same situation. The school screwed up by (allegedly) having two conflicting sets of rules.
I'd be willing to bet that the school, as is typical, will ask for dismissal of the case, (which could happen) but the judge will let it go to trial because it has merit.