「このクラスは全部で１５人なんだけど、いつもだれかは
欠席するから、みんながそろったのを見たことない」

I have 15 students in my class but ===========
I have never seen all the students are in.

この、「いつも誰かが欠席するから」のところが
どうしてもわかりません。
だれかhelpしていただけますか？？

...but there is always someone absent(s?)... 　

We always have somebody missing　はどうでしょうか〜？？ 　

There are 15 people in this class, but someone is always absent so I’ve never seen everyone here at once. 　

The class of 15 students has never been in full attendanc, because some of any students were absent in any class.