How are you determining user type? If it's by a CB field then you don't need CB Auto Actions for this at all. You can just use IF substitutions in your Pending/Welcome emails to change what the message should say. Example as follows.
[cb:if cb_type="Teacher"]Text to send to the Teacher type.[/cb:if]
[cb:if cb_type="Student"]Text to send to the Student type.[/cb:if]
More information on substitutions can be found in the below tutorial.
www.joomlapolis.com/support/tutorials/107-use-cases/18353-using-substitutions-throughout-cb
If still going to use CB Auto Actions then the below triggers should work for the various situations.
onAfterUserRegistration = After user has registered
onAfterUserApproval = After user has been approved by an Admin.
onAfterUserConfirm = After user has confirmed their email address.
onBeforeUserActive = Before user has become completely active (confirmed, approved, and enabled)