yeah that will just update that attribute for the users since zimbraHideInGal is not a valid attribute for CoS yet. Maybe sooner if you vote for teh bug uxbod filed.
You wouldn't want to run these commands every time you add a new user, theres no reason to set it for everyone each time. You just need to set it for the user so either do it through the web interface or the command prompt. You can make a easy shell script so you don't have to remember the exact syntax.
Make a new file like hideingal.sh
Code:
#!/bin/bash
/opt/zimbra/bin/zmprov ma $1 zimbraHideInGal TRUE
chmod +x hideingal.sh
then all you have to do when you add a new user is from the command prompt do
hideingal
newstudent@whatever.com
and it'll do that.