There are websites out there (e.g. RateMyProfessors) where students can post information about teachers they had.
Is reading the "reviews" on the site, or writing such a "review", a violation of Lashon Hara?
Assuming the review is accurate and constructive, it would be 'l'toeles' - for a constructive purpose - even if negative. An analogous case would be informing someone that the person they're doing business with has a history of dishonesty. That being said the Chafetz Chaim has a great deal on how to go about Lashon Hara l'toeles. The best summary I found is this hebrew page. You can also find the actual sefer here. Klal Yud deals with this topic. It's very complex and the CC goes into deep detail, so I suggest studying it inside.
As for reading the reviews that would likely fall into the category of 'public information' and be permissible. (Note that you still are required to try and maintain objectivity and be 'dan l'kaf zechus' with such information)
Also from a technical perspective the CC admits that the basis for biblical prohibitions vs. Lashon Hara are almost entirely directed at Jews, but discourages L'H against Non-Jews since it's an objective evil that should be avoided regardless.
I will cite the source later... Gd willing.
Statements which are told to people to prevent them from being harmed, do not fall under the category of Lashon Harah.
Sites such as RateMyProfessors, are a classic example of when it's ok to say negetive things about another person.
Reading the reviews on a particular professor however, is only allowed by people who attend the Universities where those professors teach, or knowing somebody who does. Doing otherwise is lashon Harah.