From site: 

Adopt-A-Classroom invites the community into the classroom in support of teachers and their students.   By   adopting a classroom, donors form partnerships with specific classrooms providing financial and moral support.   The result is a meaningful contribution to education in  which  donors experience the impact of their efforts and celebrate in a classroom’s success…

Adopt-A-Classroom partners donors with teachers so you can have funds to purchase critical resources and materials for your classroom. By registering, your classroom will be posted on the Adopt-A-Classroom website available for donors to select. When adopted, you will have full discretion to purchase items that meet your unique classroom needs.

From site:

ClassWish, a nonprofit, makes it easy for teachers and schools to create Wish Lists of the supplies they need for students to excel. Parents and others in the community see exactly what is needed and contribute online. Together, we can make a powerful difference in our children’s lives.

• Teachers visit the site to create a Wish List of the things they need to equip great classrooms, as easily as shopping online.
• ClassWish helps attract parents, alumni, local businesses and other potential supporters to see exactly what is needed and to inspire their help.
• Contributions are tax-deductible and ClassWish provides a receipt
• ClassWish sends the supplies directly to teachers at the school

Find stuff for your classroom in your local community. Register on Freecylce site for the group(s) closest to you. Then check your group periodically to see what’s being offered. This can be a good source of local items you might need in your classroom such as furniture, display stands, books, games, rugs, etc.

First Book’s mission is give children from low-income families the opportunity to read and own their first new books. To that end, educators from Title I eligible schools (and other programs serving low-income students) register with the site to get books from two sources:

• The First Book National Book Bank distributes brand-new books, donated by publishers, free of charge to programs serving children from low-income families, or for a minimal shipping fee.
• Low cost books and resources from the First Book Marketplace at 50-90% off retail price.

GSA administers the Computers for Learning (CFL) website, under federal law, to transfer computers and related peripheral equipment excess to their needs directly to U.S. schools and some educational nonprofit organizations. The CFL program specifically matches the computer needs of schools and educational nonprofit organization with excess equipment in Federal agencies.

As with most government programs, the school must go through some red tape to get registered and see what computer equipment is available. Also, the school is responsible for pickup and delivery of the computer equipment it is awarded, so location is a factor.