100 Days of School 2023 Yard Sign Letters, Happy 100th Day of School Lawn Cutouts 100 Days Yard Sign Walkway Decorations With Metal Stakes. Economical and durable, our Coroplast Yard Signs are a staple of outdoor advertising. These Happy 100th Day of School Lawn Decor Signs Outdoor Lawn Letters Yard Signs full color direct printed graphics are vertically fluted for use with Stakes. Coroplast is used for yards, parking, real estate, and election signs. They are waterproof.