Happy 100th Days of School Yard Sign - Coroplast Kids Kindergarten Preschool Primary School, 100 Day of School Yard Sign with Metal H-Stake. Economical and durable, our Coroplast Yard Signs are a staple of outdoor advertising. This Kids Kindergarten Preschool Primary School 100 Days Party Yard Sign full color direct printed graphics are vertically fluted for use with an H-Stake. Coroplast is used for yard, parking, real estate, and election signs. They are waterproof.