My son wears a uniform and it does not have a tag. The shirt is a really dark blue so a sharpie won't work. If you used tags and they were successful, can you suggest which ones you used? We tried a few from a local store and they fell off after one wash (even though they said they were washer safe).
I generally use a Sharpie, and have found that sometimes even tagless shirts have a tag in the side seam near the bottom of the shirt.
I've also heard really good things about Mabel's Labels iron on labels. I've never used those, but have loved the sippy cup labels from them, which last forever. Mabelslabels.com