真剣にやってよね。学園祭は待ってくれないんだよ、もう大詰めなんだから。

。、。

Take this seriously, okay? The school festival won't wait — we're already in the final stretch.

Literal

Seriously do-[request-てよね]. School-festival [topic-は] wait-[not-てくれない][explanatory-んだよ], already final-stage [explanatory-なんだから].

真剣にやって 'do it seriously' — a te-form request. 学園祭 'school festival' — a major annual event in Japanese schools. 待ってくれない 'won't wait for us' personifies the deadline. 大詰め 'final stage, climax' comes from kabuki theater. なんだから at the end gives an insistent, explanatory reason: 'because we're already...'