Apple Remote Desktop and Remote Management

I'll get to your question in a minute but first, in most enterprise environments the server would be in a cabinet tucked or locked away in a separate room. The only way to use ARD installed on that server would be to ARD or VNC from another mac/pc. For me this makes no sense unless your environment allows you to sit at the server itself, which in most admins eyes would be a security risk.

Install and use ARD on a workstation instead, far better, easier and more secure to do what you want.

"Is there a way to activate Remote Management on clients?"

Not unless you enabled Remote Management on the clients first. Assuming your clients were built with a deployment solution such as DeployStudio then most admins would enable this option in their build first before deploying it to the rest. Using a deployment solution - amongst other things - makes it simpler and easier to manage your mac estate. The only other way is to have enabled SSH on all the clients first. You could then use kickstart to enable ARD on all your clients. Beyond this you are really looking at walking round each one in turn and enabling it manually. Don't forget to enable SSH as well.