Difference between 'to' and 'jest' when describing people

Hello,

I wonder if someone can tell me... what is the diference in meaning between:

Kasia to mila nauczycielka

-and-

Kasia jest milą nauczycielką

(I've learnt that this one is the instrumental case, to be used with expressions like 'Jestem Polka' and 'Jestem Lekaerzem').

I'm Confused!

Thank you :)

Kasia to miła nauczycielka. - is totally OK, but it's considered more colloquial, so you shouldn't use it at school or university too much and rather choose the official version:

Kasia jest miłą nauczycielką.
but talking to your friends, you can safely use both (unless your friends are some freak language purists)

Dziękuję Krysztof!

The difference is that Kasia to miła nauczycielka is still in the nominative case because there is no verb such as the verb to be-być being employed in the present tense.