Can teachers enter marks from the staff app?
No. Marks are entered on the web, in Exams Scores, where the whole class list fits on screen with marks, maximum, percentage and a column for anyone who missed the paper. The staff app is for the work that happens away from a desk: the register, the daily progress report, class news, photos, study material, progress notes and students' questions.
Once marks are in, results, rank lists and report card PDFs come from the same records on the Gold plan. See exams and report cards for how grading schemes and locking work.