Hi,
If you just want to display the size, try using the number of attendees as your attribute to style and use counts and amounts to show the difference in size:
Style numbers—ArcGIS Online Help | ArcGIS
If you want to map by categories (school) and the number of attendees, select both attributes and use types and size:
Style categories—ArcGIS Online Help | ArcGIS
Here is a link to a quick reference of available styles:
Change style—ArcGIS Online Help | ArcGIS
-Kelly