That was a really messed up story man. I hope that didnt actually happen. If youve got you know something going on up there telling people youre gonna kill yourself isnt going to make anyone want to have anything to do with you lol. Consider not telling people youll kill yourself when apologizing for acting bizarre.
Yatsufusa
I cannot give you a "definitive answer", but I know a lot about Japan. I'll space my statements out a little, so they are easier to process.
Let me start by saying: They might not 'have' Instagram accounts. I don't have one either. Because Instagram is a site with a ton of fundamental issues...
'From how you describe the situation', you were basically strangers. Why would anyone trade contact info with a stranger?
Your letter also makes you sound overly dramatic, pretty entitled and... the way you phrased it, it sounded as if, from your own perspective, you were offering them some kind of "favor" by exchanging contact info...
Japanese school/university life is _brutal_! I don't know about Australian schools, but most European schools (including UK) and US schools are a _joke_, compared to what they go though. And those who "underperform" too much under the "expected results" will essentially end up as replaceable office drones - which most schools line their pupils up to become anyway. The social pressure from peers and parents can not be underestimated either. Suicide rates at that age are _through_the_roof_...
So yeah: I can imagine that the students who read your letter either were genuinely "upset" or saw an opportunity to escape that environment for a few hours. I do not know them, so I cannot tell you 'exactly' what went down.
The best advice I could give you in this regard would be to send a letter with a formal, neutral greeting, "I am sorry that my last letter has upset you and assure you that there will be no further contact from me." and a "Sincerely, [YourNameHere]". That way they at least have no need to feeling anxious about any future correspondence.
There are probably even better ways to phrase this, but unless you can talk to a genuine Japanese person to convey what I just said 'in Japanese', that "one-liner" letter is probably your best option. (Online translation would not do. There's more to a language than just using the right words.)