When sourcing school bags, the denier (D) count of the polyester is critical; 600D polyester is the industry standard for a balance of weight and durability, while 900D or 1680D offers superior tear resistance for heavy-duty use. Ensure the fabric has a PU (Polyurethane) or PVC coating on the reverse side to provide water resistance, protecting books and electronics. Additionally, verify the colorfastness to rubbing and light (Grade 4 or higher) to ensure the bag does not fade or bleed onto clothing.