To see student's attendance from an imported CTF, go to the student's profile. Select Attendance > Previous Schools from the left-hand menu.
This will only give you an overview of percentage marks for Present, Authorised and Unauthorised absences, along with the number of sessions they attended.
You will not be able to see a breakdown of their marks beyond this or edit the marks.
The Marks (by date) section of the student profile is for Arbor-entered marks only. Any marks from a previous school will not appear here, even if they joined your school mid-way through the year.