"The class is not easy."

Translation: そのじゅぎょうはやさしくないです。

[azureviolin]

その授業は易しくないです。

[V2Blast]

The kanji was accepted!

[Zekae]

Why is クラス not correct for 'class?'

[jungerstein]

A クラス is tens of students sitting together and listening to the same lecture.

Meanwhile, it can also mean 'level' as in 'A/B/C level' (e.g. 'This apple is B level. ') or a social class.

[Razukin]

Why is その required in the answer?

[dragonzkiller]

It's not required. There are no articles in Japanese, but the closest thing to "the" is その. But その actually means "that" (close to listener, or in general). So a lot of language learning programs like to push その as meaning "the" when it is technically incorrect.

[Lordhokage]

Kurasu has a broader meaning than jugyō. In this context jugyō is more precise. Not that kurasu is wrong. But jugyō is just 1. more precise in meaning; and 2. it's the native Japanese wago word.