Can I ask why you are having an open house in the middle of the year? Usually they are at the beginning of the year, and I would have things like copies of medication policy and forms, Immunization information, health guideline policies of your school regarding headlice, fevers, throwing up, etc.
I am sure it is an admissions open house for private schools to attract new students, normally done in January or February. I would let parents know your vaccination requirements and deadlines, med policy and what you have available (AED, etc.). The biggest reason to be there is to show parents that you ARE there. Lots of privates do not offer nurses, so your visibility is really all that is necessary. Admins probably would like to just have you on display and available to answer questions. Our admissions went up by 45 kids after I started, despite being crammed into a non traditional space.
I'm at 2 public schools and I participate in the open houses. I do intake of med forms and medications, immunization records, etc. Also there to answer questions parents may have about what vaccines are required, where can they go to get immunizations for free (I'm guessing not an issue with your population), etc.
I would be ready to talk about your role there. Do you teach health classes? Perhaps some of the visiting families have a medically fragile child so you can discuss how you are able to provide care for kids during the school day.
...good opportunity to get parents on the right road with paperwork compliance if they're going to send their kids to your school. Also it's good to meet parents in a "non-crisis" situation, before you have to make the first inevitable call home for something.