"I am forever in the kindergarten."
Translation:Tôi vĩnh viễn ở mẫu giáo.
I think that it is expressed in the Tips & notes, and là is used to identify the subject with a noun.
I am still learning this course, but perhaps I can show you an example. Like this:
Tôi vĩnh viễn là Vua của đảo. > I am the King of the island permanently. / I am forever the King of the island.