I'm out in Minnesota and Minnesota Children's has a monthly program called Clinic in the Classroom that is for free CEs. Excellent content. I've also found Children's Hospital of Philidaphia and Childrens Hopsital of Cincinnati have some school-focused education modules as well. I felt like all three of these were super helpful in looking at specific disease/behavior issues and how it translates to school.
As for additional certifications, Epilepsy Foundation has some great online programs (for free) that you can do for Seizure First Aid. There are some additional modules more focused on a school nurse's role.
I'm hoping to find a school nurse job this next fall. If you have Summer flexibility, camp nurse experience might be a good way to beef of the resume. Best of luck! :)