If they're proxying HTTP traffic usually they'll block SSH and other protocols. To work around this use an HTTPS/SSH multiplexer[1][2][3] or a simple reverse proxy to accept HTTP[S] connections and connect them to an SSH server on the backend. For the ssh client use a ProxyCommand tunneling app[4][5][6] to turn an HTTP[S] proxy request into a two-way socket, and you have an SSH connection over HTTP proxies.

[1] http://www.rutschle.net/tech/sslh.shtml [2] http://www.pond-weed.com/multiplex/ [3] https://github.com/stealth/sshttp [4] http://zwitterion.org/software/ssh-https-tunnel/ssh-https-tu... [5] http://proxytunnel.sourceforge.net/ [6] http://www.agroman.net/corkscrew/