The way DHCP works is that client does the connection requests towards server, never the other way around.. Now when a client joins a network, it needs IP settings and immediately looks for DHCP server and binds to a lease. DHCP server sets lease lifetime and client has to renew lease sooner (it does it after half of lifetime). That's when server hears from client. When client leaves, it likely leaves in the way it can't "unbind" ... wireless clients most often roam out of coverage and by the time client realises it's out of coverage of previous wireless network, it can't unbind any more because it can't contact DHCP server. If client attaches to AP with same SSID (either after some searching or if roaming between APs), it assumes it's still in same IP subnet and doesn't re-bind the lease.
So to your requirement: obviously network can't be notified immediately after client disappears. What can be done is to shorten DHCP lease lifetime ... it's set by DHCP server's property
lease-time ...
docs state that default value is 10 minutes. If you have multiple DHCP servers (because you have multiple LAN subnets), then lease times can be different. Beware that some clients don't like excessively short lease times and misbehave in some random ways.