"It's a long sentence so I'm having trouble."
Long sentences are not too often used in manga. Sentences just tend to "look" long to Japanese-learners because there are practically no punctuations used in manga.
「日数{にっすう}」 simply means what the two kanji would suggest -- "the number of days".
Thus, 「[出席]{しゅっせき}日数」 means "the (total) number of days attended (at school)".
"We have a meeting next week."
"(We will discuss whether) you have attended school for a sufficient number of days."
"and whether you have not received too many F's."
「赤点{あかてん}」 is a school slang for "Fail".