"Gwers ydy hon ddim te parti."

Translation: This is a lesson, not a tea party.

[Malwen]

Why is it "te parti" and not "parti te"?

[ibisc]

It is a borrowing from English. parti te is also acceptable.

[sdmohan]

Is this a common phrase?

[rmcode]

It's the sort of thing a stressed teacher in North Wales might say.