How to SSH an outside server from a computer which is behind a proxy firewall?

I access the Internet through an HTTP proxy firewall at college. And I need to login to a computer, via SSH, which is outside our network. I tried it as Linux command and on Windows using PuTTY. I also configured PuTTY to use our server's address. But still, "Proxy error: 403 forbidden" pops up. They must've blocked SSH access to outside systems. (college systems as accessible).

I can SSH a web server (not the proxy server) at the college, which I use to browse proxy-free by tunneling. Now this server allows to browse restricted sites, but still no SSH.

Any workaround, please?