we have setup a diskless cluster where there is a server which run DHCP and there are some clients that receive an ip address from the server upon the boot.
The problem is, if the nodes (server + clients) restart unexpectedly (power plant problems), then the server will start dhcp service with delay because the fsck is running. At the mean time, the clients quickly pass the POST process and they want to get IP address from server. The server still has not started the DHCP service, so the clients stuck at the first step of network boot.

My questions:
1) is there any way to prioritize the services on server? For example, I want the DHCP to be the first service!!

2) Currently the clients wait for about 10 seconds to get an IP from server. I mean the | / - | \ progress bar. Is there any way to increase this short timeout?

fsck runs at a lower level than a service like dhcp and MUCH earlier in the boot process ie specifically during the disk mount process.
You'll have to wait.

What about a network controlled power outlet for the nodes. Hence, after the server is up, you can switch on the nodes from the server.

NB: Do you have any message on the monitor for the BIOS of the network card? In this menu for the network card, you could try to adjust to reboot if there is no response instead of falling to the next boot option (this is not possible with all cards though).

What about changing the S numbers in /etc/rcX.d/ to change the priority?

no

Your setup is buggy

Your clients have to wait; you need to switch your clients on once the server is up (as indicated by Reuti). I don't expect that the startup sequence will make a serious difference for your problem.

You need to read up on the boot process, as your suggestion in post#4 makes no sense.
fsck runs BEFORE the disks are mounted; no way you can run a service until the code it depends upon is available from the disk being mounted.

Currently all nodes (clients and server) are plugged in power source. So in case of power problems and recovery, all nodes start at the same time.
What I understand from your replies, is that adding an external device to delay the power current for the clients. Right?
Something like this:

power source -> server
power source -> delay circuit -> clients

Yes

And the nicest solution would be if you don't use a delay circuit but a device controlled by the server (serial port, network); once the server has brought up the dhcp daemon, it can start a process to control that device to power on the clients.