Contrary to Steve''s experience, I've often heard senior (and freshman, sophomore, junior) to refer to students in college as well as high school students. If you google the phrase 'college senior' you'll see numerous examples. The Wikipedia definition also describes it as referring to both high school and college/university students. In addition, it's often used alone as a noun. The seniors will have their graduation ceremony on June 15th. https://en.m.wikipedia.org/wiki/Senior_(education)

In my experience as an American, the term senior, as applied to a high school student, means a 4th year student, in the last year before graduation. We also use the terms freshman, sophomore and junior for 1st, 2nd and 3rd year students. We do not use these terms for college students.

That depends entirely on the country and the culture. Here, it would only refer to high-school students, and would normally only be used collectively, e.g. 'senior students from xxx school will be presenting a play...' (and I think you mean 'a' senior student?)