As assistant teacher at the playgroup

[lagoo]

#1 As assistant teacher at the playgroup, you should be responsible for their safety.
#2 As an assistant teacher at the playgroup, you should be responsible for their safety.
#3 As the assistant teacher at the playgroup, you should be responsible for their safety.

Which one is correct? Or maybe which ones are correct?

 

[emsr2d2]

If the person's job title is "Assistant Teacher", #1 could be used without an article but with the capitals I used.
#2 is OK if there is more than one assistant teacher and the person is just one of them.
#3 is OK if the person is the only assistant teacher.

I'm not sure about the use of "should" though. Surely an assistant teacher either is or is not responsible for "their" safety (whoever "they" are - I assume it refers to the children who go to the playgroup).