Plenty of styles/systems have tanto training. I don't know of a specific school that focuses solely on Tanto training though.

Tanto training is a major part in Aikido training. That would be a good place to start. I believe most legit Ninpo schools (e.g. Bujinkan, Genbukan et al...) also do tanto training.
"Let your food be your medicine, and your medicine be your food" Hippocrates.