The answer is pretty simple:

They are better and will last longer.

Any teacher will tell you that the knock-off brands have to be replaced MUCH sooner than the name brands.  Most schools do not have supplies closets to turn to anymore so the teachers have to pull from their own pockets to restock the class supply.  Elmer’s Glue, Fiskars Scissors and Crayola Colors(markers, crayons, color pencils) are all proven to last longer than most generic brands.

click here to see a complete list of supplies per grade or a general list with average number of items requested.

http://www.back2schoolfestival.org/school-supply-lists/

IF you can afford to, please consider purchasing better brands!