Formal:
* Stationery
* Educational materials
* Learning materials
* Classroom supplies
Informal:
* School stuff
* Back-to-school gear
* Class supplies
* Writing tools
Specific:
* Writing utensils (pencils, pens, etc.)
* Paper products (notebooks, folders, etc.)
* Art supplies (crayons, markers, paint, etc.)
* Science supplies (beaker, test tubes, etc.)
The best word to use depends on the specific situation.