In the US libel and slander are the two types of defamation, libel is written, slander spoken. The elements of either are an intentional false statement of material facts that results in damages. It must be a false statement of fact, not opinion. Thus, to state “we think these two guys are evil, drug-using Canadians” would not be defamation because it is couched as an opinion. Material means it is important fact. Thus, to mistate his hair color would not be defamation (to call them Canadians when they are not might be ). In this case the only facts stated in the ad are that the school terminated their employment and descriptions of their passport numbers. If that much is true its not defamation.
But it then says “Spontaneous [language school] is not responsible for anything they might have done outside the school.” That’s not a statement of fact about the two guys, but it certainly suggests, especially in conjunction with the termination, that they did something bad. But I believe defamation requires an express, not implied, statement of fact.
In the US I am sure there are other possible actions besides defamation that might apply, including intentional (or negligent) infliction of emotion distress and other theories that I can’t think of right now. Maybe Tigerman or some otehr lawyer can help out.
The above is based on US law, but Taiwan law of defamation is probably very similar, although its how its applied by judges that differs. Taiwan’s judges in my opinion are even more erratic, unpredictable and mistaken about the law than most judges, in part because many of them are young kids (in the US one usually has to be a lawyer for a number of years before becoming a judge; in Taiwan most judges have never been a practicing attorney).
Finally, the above description of the law all concerns liability. Even if one establishes liability, one still must prove damages in order to recover anything in a lawsuit. Damages such as pain and suffering, emotional distress, not to mention punitive damages, are extremely difficult to recover unless you can also prove some actual damages, ie. employers refused to hire me because of the ad.
In light of the above, I doubt there will be a lawsuit, much less a judgment against the school, but for the victims some other retaliation might be appropriate. He he he.