That said, since a student's main duty is their studies, neglecting the end-of-term test will get you a 'no no,' okay?
Literal
However, student [gen-の] main-duty [topic-は] studies [because-ですから], end-of-term test [obj-を] neglect [if-たら] tsk! [copula-ですよ]?
本分 'one's proper duty / one's main calling' — a slightly formal compound. おろそかにする 'to neglect / treat carelessly / slight' is a stock set verb phrase. メッ is the interjection used when scolding a small child or pet ('no no / bad'), rendered with emphatic gemination. The whole sentence mixes formal/educational register with a childish scolding interjection for comedic effect.