≫'everybody IS doing something' tez za poprawne.
I would go even further than just ‘tez za poprawne’ ( given that we stick to the strict rules of grammar, which is actually the the domain of teachers with their hands-on and cemented experience , is it not ? :)
With ‘IS’ it is correct in the first place with the other one being (squeamishly) admissible in the second ( as those T-shirt taglines).
The latter can’t stand alone as a ‘fully licenced’, complete independent sentence.