Sharpie brand markers may not be totally ideal, as they can eventually wash out, but they are vastly superior to ballpoint pens for this. I've had ballpoint pen ink disasters that washed out quite well in the laundry, exactly the opposite of what you want in this case. Use only black Sharpie pens, not the colored Sharpie pens, as the latter tend to wash out more quickly. When my sons have brought home class t-shirts that were signed with black Sharpie pens by their classmates on the last day of school, the markings seemed to hold up pretty well in the laundry. It is best if the Sharpie pens marks can dry on the shirts for a week or more before getting the shirts wet to wash or dye them, but that may not be practical in your case.

It is better, if time permits, to get a marker specifically made for fabrics, such as Identipen or Setaskrib fabric markers. A good crafts store will probably have several different choices. Some may require heat-setting by ironing to make them permanent, so be sure to check for any manufacturer's instructions. It is probably best to write on the shirts only after they have been dyed, washed and dried, but it should work okay to do the writing first, before dyeing. Tell them to be sure to turn the shirts inside out so the writing does not wear off as quickly in the dryer. (You can easily tie-dye shirts that are inside-out - it makes no difference.) I expect you'll send home the dyed shirts still wet, in plastic bags, with instructions to wash them out the following day.

Do not use all-purpose dye, such as Rit brand dye, for room temperature tie-dyeing! It is a very common mistake, but it's very sad when much of the dye washes out afterwards. Instead, you need to use fiber reactive dyes. Get some Jacquard Tie-dye kit (not a Rit tie-dye kit!!), if you can find them in your local crafts store or fabric store, or several Rainbow Rock tie-dye kits, or buy many packets of "Dylon Permanent" dye. If you have enough time, order a Procion MX tie-dye kit from one of the dye suppliers listed on my page of Sources for Dye Supplies Around the World, at http://www.pburch.net/dyeing/dyesources.shtml; these kits are typically far more economical per student and gives excellent results. (While you're at it, order some good fabric markers from them, for writing on the shirts.) Also, please be sure to buy only 100% cotton t-shirts to dye, as polyester will not take the dye, and avoid stain-resistant t-shirts at all costs.

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