Cathy, I need to know if the tile is vinyl leno tile or ceramic tile?
You did not mention this. If its vinyl tile, you have a real problem. The urine has probably worked its way under the tile and very difficult to remedy. I don't imagine that it was leno tile since your previous has used bleach on it.
If it is ceramic tile then we have a more workable solution.
Ceramic tile by nature is non-porus. the glazing process over the tile creates a surface that will not allow anything to absorb. The grout is another subject. The grout is porus and will absorb stains, odors, and stuff. When I install ceramic tile in bathrooms and kitchen areas, I always insist on using silicon sealer on the grout areas. This process has to be repeated periodically.
Sealing old tile and grout areas are hard to do once age sits in. If you seal the old grout with stuff already penetrated into the grout, your just covering up something that will later grow and stain and continue to smell.
Therefore, I would suggest at this point, you continue to use the bleach and let it sit on the surface for awhile to soak in. This means doing this after school hours.
I would also contact some industrial sites like factories, prisons, and other facilities that have and maintain big bathrooms. they may have a better answer.
Post some odor masking chemicals on the ceiling wall areas. Install a automatic timed mister in the bathroom like some hotel bathrooms have.
I would also recommend that you treat the bathroom areas daily. I am sure that you get quite a few careless users in that area.