I see both sides of this situation. There could be serious legal issues if you accuse this guy of being doped up at school and he gets ''the boot''. You also risk the wrath of the chef, who may think that you are trying to do his job, by playing supervisor. On the other hand, if you can go to the chef and cite concrete examples of improper behavior, technique, etc. you may point the chef in the right direction to watching this kid and catching him. If this chef has spent much time in the industry, outside of school, chances are he has seen, engaged or had to deal with such behavior, and will tune in to it with just a gentle nudge.
If you stay in this industry for long, you will have to deal with this kind of thing many more times. This business has a long history of drug use and abuse, and not just in lower class restaurants. Some of the hottest, trendiest restaurants I knew in Chicago had kitchens full of dopers and alcoholics, from the chef on down.