Why is it was? Shouldn't it be deshita then?
The past-tense form of an い-adjectival is made by dropping the い, and replacing it with かった, followed by the predicate 「です」. Since the adjectival itself is in the past-tense, there's no need to do the same for the predicate.
Nope, because it conjugates おもしろい to おもしろかった. Past-tense い-adjectivals are never stated in their plain form followed by でした.
Could "that teacher's lessons were interesting" also make sense?
Can someone tell me how 'class' was pronounced here cuz it sounds odd to me
授業 is pronounced じゅぎょう ju gyou. The u sound in ju should be short, and the o sound in gyou is long.
I think the audio for this exercise is correct, but it's just so fast that the ju gets even shorter and sounds more like ji, which is common/normal for native speakers.
How would it be: "that lesson of the teacher ..." ?
"Ano sensei no jyugyou" I think
That's what we already have in this particular sentence though...
That's because it reads both ways, I'm pretty sure.
The class of [that teacher]
[That teacher]'s class
Isn't 先生 a treatment word? When you're referring to someone who's a teacher, you'd use another word, isn't that right?