。
You'll have to study even harder next year.
Literal
Next-year [topic-は] more hard-ly study-not-must [emphasis-よ].
一生懸命に ('hard / desperately') uses the explicit adverbial marker に on the four-character compound 一生懸命 — 一生懸命 floats freely as adjective-like or adverb-like, and the に form sounds slightly more emphatic. Combined with もっと ('more'), it doubles the intensity: 'study even more desperately.' ~なければいけない is the obligation form.