"This class is very fun."
Translation:Esta clase es muy divertida.
Don't think of this as an issue of temporary vs. permanent. Perhaps a better way to determine which verb to use would be to decide whether this is a quality/characteristic of the thing being described or a condition/status. If the intent was to say today's class activities are very fun, then you could use estar. (Although, in that case, you'd probably phrase it differently.) If you mean the class in general is a very fun class, then ser is the proper choice. Most of the time, I assume, we mean the latter and that makes ser the more common/likely verb to use.