Could 恭喜发财 sound inappropriate?
恭喜发财 may not be the best greeting word from student to teacher, but it's not awkward - the intention is always good. 

For example, parents usually wish children with good health, striving and studying. If a parent greets children with wealth, you may find them funny, fashionable or friend-like, but definitely not mammonish or evil. 

In old Chinese traditions, there were strict rules about etiquette and word usage. A small misuse can be considered a big issue. But that time has long gone.

It might sound a little sarcastic if you say 恭喜发财 to someone who is obviously in an opposite situation, otherwise it should be fine to say it to anyone.