General:
* School social
* Dance
* Formal (if it's a more formal event)
* Prom (if it's a high school prom)
* Hoedown (if it's a country-themed dance)
* Ball (if it's a particularly grand event)
More specific:
* Winter Formal
* Spring Fling
* Homecoming Dance
* Junior/Senior Prom
* Back to School Dance
You can also use phrases like:
* The school's dance
* The dance at [school name]
* [Grade level] dance
* [Theme] dance (e.g., "The 80s Dance")
The best word to use will depend on the specific context of the dance and the level of formality you want to convey.