Grey area, as its both possible that your son isn't guilty of sexual harassment and that the administration is not being overly sensitive with this matter. What your son said sounds fairly innocent and you as his caregiver would probably know that best after talking to him, at the same time you can't account for all the other children that attend the school or grant that a teacher will be as insightful about your son's tone or meaning. Hypothetically a child in a similar position could have said "I like her bum" and have as much innocence potentially, it is just another human body part, or can be to individuals, just adult society at large generally tends to be aware that part of the body is a bit more sexualized. Then with that in mind a lot more adults would generally feel a little bit uncomfortable or perplexed at children talking to them that way.
If I were in this situation, I would probably call or met with the administration and teacher and just explain that I talked to my child and that what they said was sincerely innocent, and that they don't quite realize what all the fuss is about, but that I also spoke to them about what can be appropriate and inappropriate ways to address others and that you can understand if the teacher may have felt uncomfortable in such a situation and apologize on behalf of my son.
Do you know if the other children's remarks were addressed? The ones who said they liked her hair and pants? That could be a factor as well. I have a sister who I essentially raised as a daughter, she is 12 now, a few years ago had to talk to her about swear words, since I discovered kids in her class were started to share this secret knowledge. I am one of those a word is just a word types, and so my sister has a similar, simpler approach as well (which helps her as far as others bullying or teasing her) but she also had to understand that certain ways and addressing other individuals can be considered inappropriate even if her intent isn't inappropriate, and how thats important as far as real life as she may not always interact with individuals or groups of people who have the same attitudes or approaches to life and words as her, so its important to always be mindful of the context of a situation and to help avoid offending people, context dependent. The ability to discern context at a young age isn't necessary the greatest so I tried to instill to my sister that its better to approach people being a bit more polite until she gets older and can be more choosy about how she chooses to address others.
Good luck either way!