"That class is not easy."
For easy we have 易しい (やさしい), but there is also the noun and na-adjective 簡単 (かんたん). Can that one also be used?
My understanding is that 簡単 is more like "simple" or "brief" or "uncomplicated" and 易しい is more like "easy to do" or "user-friendly" or "simple to understand" So the opposite of 簡単 would be "complicated" and the opposite of 易しい would be "difficult".
They are similar and there is some over-lap in how they can be used, but they have differences in nuance. Sometimes, they cannot be used interchangeably, because something might be easy to do, but not really "simple". Or something might be very simple and uncomplicated, but very challenging to accomplish.
Sometimes something is both uncomplicated and not very challenging. In that case, you could describe it with either adjective, but you are describing slightly different aspects of the thing.
[romaji] Sono jugyou wa yasashikunai desu.
優しい means “nice”. 易しい is easy.
This question is in the present tense. You can say 易しくないです (yasashiku nai desu), which is the expected answer above.
If you wanted to say it in the past tense, you would say 易しくなかったです (yasashiku nakatta desu).