If you miss school more than a certain number of times, you won't be able to move up to the next grade.
"If you miss school"＝「もしあなたが学校を欠席したら」
"more than a certain number of times"＝「ある回数以上に」
"you won't be able to move up to the next grade"=「進級することができなくなります。」
Sentence in context:
"School attendance can be strict in Japan. It you miss classes more than a certain number of times, you might not be able to move up to the next grade."
アメリカ合衆国 
