Maybe your child has started school and emulates it at home. Or perhaps a younger sibling plays school at home in anticipation of one day attending themselves. Either way, it's something we've all probably done and you can have things on hand to encourage the fun:
Chalkboard: Having a chalkboard on hand used to be a big deal. We had one at home when I was a child, but it was only about 2'x3'. With today's trend of chalkboards in decor, you just might have an entire wall in your home for your children to use when they're playing school!
Desk: This can be anything from a secondhand school desk (vintage ones like this can be had pretty inexpensively on Craigslist) to a child-size table and chair to sitting on the floor at the coffee table.
Supplies: We always enjoyed having a wand of some sort as the "teacher," used for pointing our lessons out on the board. It's also fun to have chalkboard erasers just like the kind you pounded out on the recess play yard (that was a reward, not a punishment, where I went to school!). Writing utensils and paper for the "student(s)", or course, are needed. And the perfect finishing touch? Old text books, which can be purchased for a song at school library sales.
Do your children play school at home? What else do you have in your setup to make it believable for them?