[CentOS] Samba share configuration: r/w for some users, read-only for others

Hi,

I'm using Samba for some basic setups like open shares for everybody, or 
restricted shares with Samba users and passwords. So far, everything 
works fine.

I have two situations which both more or less amount to the same, and I 
don't know how to configure it. One is in a school, the other one in a 
town hall.

1) In the school, I'd like to configure a share that is readable and 
writable for all the teachers, and read-only for the students. The idea 
is that the teachers can write some assignments or various papers and 
then put them in the share so the students can access them (without 
modifying them).

2) Similar setup in the town hall. The mayor wants to have a share where 
he and his secretary can fully access the documents, write them and 
modify them, and the library users can then access them read-only.

Is this feasible with Samba? And if so, what would the according 
smb.conf look like?

Cheers from the sunny South of France,

Niki