But on mock-exam days, club is cancelled and I get to walk home with Haruka, so I'm actually grateful — me, I mean.
Literal
But mock-exam [gen-の] day [topic-は] club-activity [subj-が] holiday [become-になって] Haruka [with-と] can-go-home [and-し], am-grateful [though-けど], I [topic-は].
The displaced topic 僕は appears at the very end of the sentence as an afterthought — a casual/emphatic word-order pattern in spoken Japanese. The chain 部活が休みになって...帰れるし...感謝してる builds reasons for the speaker's positive reaction to mock-exam days, with ~てなる (become) and the potential 帰れる ('can go home') tied together by ~し. 模試 is a compact abbreviation of 模擬試験 'mock exam.'