When choosing websites and online resources for classes, a certain amount of preparation and personal judgement is required.
Under no circumstances should resources be presented to a class without the teacher having visited and screened the site for content and structure. Kathy Schrock has an ABC's of Website Evaluation on her website that can provide a good basis for website review. Her list is quite comprehensive and would certainly reflect an ideal resource. Practically, the classroom teacher will need to use personal judgement and weigh benefits against detractions.
Generally, the teacher needs to be aware of additional links, videos, and even advertisements that could be inappropriate for their students. Schools do have filters that block a large amount of inappropriate material, however, things always have a way of sneaking through. Obviously, the content needs to be appropriate and relevant for the lesson that is being explored.
Depending on the age and level of the class, students will percieve information very differently, so care needs to be taken to insure an appropriate response to the resources. For example, if you know that students are particularly sensitive to certain topics, that might not be one that you would want to explore in the less controlled environment of the Internet.
Personally, since I teach older and higher level students, I tend to have expectations of them similar to those of responsible adults. They understand the sometimes unpredictable and inaccurate nature of information that is gathered online. Our school does an exploration of website reliability in the Freshman Seminar class that all freshmen take, and all teachers encourage and model corroboration of information from multiple sources. So, website review has become a bit of the culture that the Internet and Web 2.0 has created, and it should be a consideration within the technology in the classroom.