I am so sorry, I didn't receive a notification of your answers. It's late, but I'll answer anyway ^^
Regarding your message to write them back, starting with ε
ηζΉ seems wrong because you're answering to students and ε
η means teacher.
ηεΎζΉ is not something I have heard of, and it seems waaaay too strict to address to young people.
Regarding the tone of their message, it's very friendly and is "slangish" even. γΎγγγ instead of δ»»γγ¦γγ γγγ
If you want to answer in the same vibe, I would simply say: γγγγγοΌΎοΌΎ or something along those lines.
For something more formal: γεεγγγγ¨γγγγγΎγ would be good IMO.
Note: I am not native Japanese, take my advice with a fistful of salt ^^