If you're looking for a way to make your child's hair grey for 100 days of school, there are a few options you can try. One is to use temporary hair color. This will allow you to change your child's hair color for the duration of the school year, and then return it to its natural color when they're done. Another option is to use semi-permanent hair color. This will also allow you to change your child's hair color for the school year, but it will fade over time so you'll need to touch up their roots every few weeks. Finally, if you want a more permanent solution, you can consider using bleach or Highlights. However, keep in mind that these options can be damaging to your child's hair if not done properly, so it's important to consult with a professional before proceeding.