Student or Learner
As I read some articles on the Internet, I came across the same phrase/nouns but written in a different manner.
1. student achievement- students' achievements.
2. student engagement - students' engagements.
How these two varieties are possible?
The apostrophe is a difficult part of the language to understand: it denotes possession. So students' ---would precede what is pertaining to students, so students' engagement would be the engagement pertaining to students. Student's engagement would be the engagement pertaining to a single student. To have an apostrophe the noun must be definite. Indefinite articles do not have apostrophes.