Can be done - little bit of scripting magic needed to add it to all students.
First, create a new group - call it 'Can E-mail Students' or simelar, also make sure you create an e-mail address for the group or the next step won't work.
Next, look at the AD properties for a user, must be on a machine with Exchange management tools installed. Look at the Exchange general tab, and click the button 'delivery restrictions' and down the bottom you can set the restriction of 'Only From' and then add the group you previously created. When set, the student will only be able to receive mail from members of this group - so if you add all your staff to it, then all staff can e-mail students but students can't e-mail each other.
But wait, I hear you cry, I've got to make this change to each user individually?
Actually no, I've written a VBscript which will do that for you.
You need to specify the group you've created at the top as an LDAP string, same for the OU you want to target. You can get the LDAP strings by using LDAP browser and copying and pasting them from there.
Const strGroup1 = "CN=Student Mail,OU=Network Operational Groups,OU=Groups,OU=Spires Academy,DC=SPIRES,DC=LOCAL"
strOU = "LDAP://OU=YEAR07,OU=Students,OU=Users,OU=Spires Academy,DC=SPIRES,DC=LOCAL"
Set objOU = GetObject(strOU)
objOU.Filter = Array("user")
For Each objUser In objOU
objUser.dlmemSubmitPerms = strGroup1
wscript.echo "All Done"
Simply repeat the block of code between "strOU = ... " and "Next" for every OU you want to set delivery restrictions on.
Works like a charm here.