How to address fellow students junior to oneself (学妹，学弟)?
To be honest, although you could use 学弟, 学妹, etc. to address other students, the most natural way of addressing them (especially when you interact face to face) is by full name. Full names retain a reasonable amount of respect without making it sound too formal. Adding a suffix would only make it overly formal and consequently awkward. For example, full name + 同学 might be used by a teacher when announcing an award given to the student.

Side note: 兄，哥 were used in more ancient times such as 师兄 and 师哥. Back then things tend to be much less casual. For example, if you and your older "classmate" both train under the same martial arts master, you'd might call that student 师兄.

Reference: I grew up in China.