I am not sure what the description of Magic Jar means by “class levels” and “class features”:
[…] Once you possess a creature’s body, you control it. Your game statistics are replaced by the statistics of the creature, though you retain your alignment and your Intelligence, Wisdom, and Charisma scores. You retain the benefit of your own class features. If the target has any class levels, you can’t use any of its class features. […]
For example, if I possess a Gnoll, do I gain the benefit of its Rampage feature? Is that a class feature?