Learn Hardcore Korean: hakgyoga jaemiisseoyo.

가 is the subject-marking particle. It highlights that 학교 (school) is the specific subject you’re talking about. If you used 는 (the topic particle), you’d be shifting to a more general or contrasting statement (“As for school, it’s fun…”), whereas 가 simply presents 학교 as new or focused information.

What part of speech is 재미있어요? Is it a verb or an adjective?

In Korean grammar, 재미있어요 comes from 재미있다 and functions like a descriptive verb (형용사). Descriptive verbs in Korean are often called “adjectives” in English, but they conjugate like verbs. Here, 재미 is the noun “fun/interest,” and 있다 means “to have” or “to exist,” so literally it’s “fun exists.”

Why is there no pronoun like 저는 (“I”) at the beginning?

Korean frequently omits subjects and objects when they’re clear from context. Since it’s understood you’re talking about your experience with school, you don’t need 저는 (“as for me”). If you wanted to emphasize “I,” you could say 저는 학교가 재미있어요, but it’s not required.

What politeness level is the ending -요 in 재미있어요?

The -요 ending marks the polite informal style (해요체). It’s appropriate for everyday conversation with people you’re not extremely close to—strangers, classmates, coworkers, etc.