How do I enable ntpd to serve ntp clients (using High Sierra)?
You have to configure ntp.conf and ntp-restrict.conf to run a local  time server:


Kill ntpd
Add 

server 127.0.0.1 prefer
fudge 127.0.0.1


at the end of /etc/ntp.conf.
Add 

restrict &lt;network_address&gt; mask &lt;net_mask&gt; nomodify notrap


after the localhost restriction block in  /etc/ntp-restrict.conf. Replace &lt;network_address&gt; with your local network address (e.g. 192.168.2.0) and &lt;net_mask&gt;with your mask (e.g. 255.255.255.0)
Reload org.ntp.ntpd-legacy (I had no problem to load the system daemon - SIP disabled though):

sudo launchctl unload -w /System/Library/LaunchDaemons/org.ntp.ntpd-legacy.plist
sudo launchctl load -w /System/Library/LaunchDaemons/org.ntp.ntpd-legacy.plist

Check your time server locally:

ntpdate -vdq 127.0.0.1


Usually the server is not suitable on the first attempt because the stratum is to high (i.e. 16 = unsynchronized)! Wait some time and try again. Here it took about 10 minutes to bring the stratum down to 2 from 16.
Connect your Mac to the private network and try to sync your Raspberry Pi clients.