This often happens in schools , colleges and now some offices too lol , that some of the sites are blocked . This is how you would bypass it

Bypass by using a proxy

If the proxy sites aren't blocked then it shouldn't be a big deal to gain access to those sites by using a suitable proxy . There are many sites to look for fresh proxy lists , one of which is : www.samair.ru/proxy

Bypass by using IP address of Site

Now what if they have blocked all the proxy sites as well . In that case DOS comes handy . You just need to use the ip address of the site instead of its name to open it in the browser . This works in most of the cases .
So , how do you find the ip address of the site ?

Method 1
Open DOS by typing cmd in run) then ping the site
Example , ping www.google.com
In the first line of the result : pinging www.l.google.com [ ip address ] with 32 bytes of data , you will get the ip address .
Then just use the ip address of the site in the browser .
Method 2
This one is easier .
Type in DOS , nslookup www.sitename.com
This will give you the ip which maybe used in the browser to open the site .

Hope this helps !