My Pi has Raspbian Jessie and it is behind my college's proxy server. I am able connect to the internet and download packages.
I even tried doing export http_proxy=user:pwd@proxy:port
I am not able to access any website using Chromium browser.
Changing the proxy settings is in chromium is normally not necessary since they are picked up from the system.
However, assuming you have already rebooted and restarted chromium, and it still is not working. You can configure the proxy settings for Chromium by clicking the menu (three vertical dots to the right of the address bar), and selecting settings. Then scroll to the bottom of the page and click Show advanced settings. Under the Network heading click Change proxy settings You can also configure the settings from the command line. Use the following command:
to get the specifics.