You might be able to do everything from the server.

You can wake the server at scheduled times easily enough from the sys prefs.
You can schedule a script to run using cron.
This script would need to contain lines to accomplish the following:

01 Wake the first client machine;
02 Mount the shared volume;
03 Copy the user folder to the server;
04 Sleep the client;
05 Repeat the first 4 for the other clients;
06 Sleep the server;

Lets address these one at a time (If I could write the script for you on the fly I would, but my scripting isn't that hot).

01 You could wake the client machines using sys prefs too, but you would need all machines to be consulting a network time server and/or some variation in waking times to allow for services and things to startup and you are going to end up with machines waiting for each other to finish backing up unless you do them all at once and either way, you are wasting electricity.
Better to use magic packets to wake the clients. I use an app called Wake Up! to do this from the Finder, not sure if it is scriptable or not but I would be very surprised if there is not a command line tool which can do this part for you. Should be able to put them to sleep again too.

02 It should be easy enough to mount the client. I imagine the Mount command will do it for you, you'll have to look up the exact syntax.

03 Copying is easy enough. the copy command in terminal is cp.

04 As mentioned, your magic packet tool should sort this part out for you.

05 You could try and be fancy and use some kind of loop, but since you only have 5 machines, you may as well make good use of copy and paste and just change the names of the clients and user folders as needed.

06 I imagine there is a simple command to do this, but I don't know what it is. I know there is one for shutting down.

I mentioned that some services will need time to start up. If you wake a client and immediately try to mount it over afp, you are risking timeouts. You can make the script pause using the sleep command. (Syntax is "sleep n" where 'n' is the number of seconds).

Doing things this way will require the clients to run AFP of course, and to sleep at all times instead of shutting down, but to be honest only the RAM is powered during sleep and that runs on 2 or 3 V, so its not so bad.

The only problem I can see doing things this way, is that when you copy a user folder from a client over AFP, you tend to get permissions issues. That should be the trickiest bit.

macosxhints and the o'reilly website are good sources for learning about terminal commands. If you know the name of a command you can read its instructions using the 'man' command. ("man command") where command is any terminal command will display the manual pages. If youa re looking for a command, you can search for keywords using the 'apropos' command, followed by a keyword to look for. This will show a list of commands which are relevant to that keyword.

You will find there are good scripters on the Apple Discussions boards if you want someone knowledgable to ask about things.