1) When the teacher gets in the classroom, for example, what does she/he say? (answer：同学们好) 2) The teacher talking about her/his students : 我的学生们 would be right? (answer: right. But 我的学生 is better) 3) When I approach a group of friends, is it better to say同学们好 or 学生们好?(answer: 同学们好) 同学们：students who study together. (although teachers teach students, but usually, teachers are humble to say “let’s study together”. So, the students feel close to teachers. ) 学生们：students (just students and teachers, not feeling close) Other samples: To the teacher, 1) When talk to another teacher, the teacher will say “学生们学习挺努力的” or “同学们学习挺努力的” 2) When talk to students, the teacher will say “同学们学习挺努力的”

A的学生 means A's student(s) A的同学 means A's classmate(s) However, it's unusual to ADDRESS a student as 学生 or a group of students as 学生们, even if you are their teacher. Instead, we usually say 同学 or 同学们. e.g. "老师好!" "同学们好!" "同学, 你好, 请问5号教学楼在哪儿?" (When you are looking for building no. 5 in a school and see a student nearby, you may ask him or her like that.)

Hi, man. “同学“ is a kind of relationship. If you and someone study in the same class (even the same school), you can say he or she is your 同学. When you introduce your calssmate to someone else. You can say: 这是我（的）同学。 Senior high school classmate is a very very important relationship in China. When you are in a collage, you can call those students "同学"。 eg: 同学，请问图书馆在哪里？ "学生" is a kind of job, do nothing only study O(∩_∩)O~. eg: What are you? 你是干什么的？ I'm a student. 我是学生。 Just like kids belongs to their parents. Students belongs to their teacher. So if you are a teacher, when you introduce a student in your class to someone. 你可以说：这是我的学生。 But if you are a teacher, when you enter the classroom, we don't say 学生们好！, we say 同学们好！ instead. That maybe weird for you, that is unique.

1。"同学们好!"， 以表亲切慈爱。 2。 "我的学生(都很优秀)!"， 不要用"们"。 3。 "同学们好!"。 在现实生活中，说"哈喽！"最好。

同学 = classmates/schoolmates 学生 = students/pupils. So the teacher using 学生 as in your examples are correct. If uttered by a student, 学生 is used as the third person or that the entire group is referred.