All published work/information available on the internet is applicable to copyright laws in the United States as well as in other countries. Therefore, under the rule of thumb, better safe than sorry, it is absolutely imperative that you follow copyright laws.
Whether a web page has a copyright statement or not, all copyright laws apply. Therefore, please obey the following:
Obtaining permission for images to be used on your school web page is often very simple. Look for copyright disclaimers on the website by going to the website's homepage (See Exploring Domains for information about getting to a website's homepage). Often, permission is given in these disclaimers for non-profit or educational purposes. Sometimes the authors will request that a statement and link is provided on the published work back to the originating web page.
If no such disclaimer is available, a quick email to the webmaster asking for permission is often successful. If no email address or disclaimer is available, it is a good rule of thumb to always give credit where credit is due. Therefore, on your published work, give a reference to the location where you image/information was obtained.
Lastly, the copyright information above is not at all a synopsis of all the copyright laws that are applicable to information on the internet. However, they are simply guidelines that we suggest educators keep in mind when using the abundance of resources on the internet for their classroom use.