Years ago, rather than painting things onto the field, we went to the hardware store and bought the silver, thin circles that were about 2" in diameter (can't remember the exact name of them).

After that, we gave each student 5 or 6 in a baggie that had been spray painted different colors that they were responsible for keeping up with. Then we would assign each set a different color. After 5 or 6 sets, we would just reuse them for the later sets.

That way, we didn't harm the field, but we could easily tell which set it was because of the colors and they could pick them up once they learned the sets and wouldn't rely on them.