It depends on the reason, some I can understand but for most I really judge upon.
If it's a family emergency or bad accident etc., then yes, although I'm upset about it I can understand. However it has happened to me before and the reason provided to us was that the teacher just quit teaching because he didn't feel like it anymore. That, I really can't understand and was extremely angry about it. It was irresponsible and selfish. Luckily my new teacher is a fantastic teacher, better than the previous. Also another teacher left for a term because she decided to go on holidays.
It's interesting because back in Year 10 I had a teacher who got informed mid-year that his application for a one-year paid holiday leave was successful and he can leave straight away. He wanted to but he chose to finish the year with us and then go on his travel around Europe. You'd think teachers who has HSC classes will be much more considerate and thoughtful than Year 10 teachers ahahaha.
Anyway advice is work around it. For a while I was really down and complained about it a lot, but at the end I realised it doesn't really change anything. What's done is done. What you can control is your attitude towards the event and hopefully everything will get better from there. Life isn't all about what you want, and in life you just have to deal with these type of situations (sudden departures, communicating with individuals you don't like etc.)
Having said that, from a student's perspective I'm not happy about it no matter what the reason for leave is.
I understand, but I'm never happy with teachers' sudden departures, unless they're that bad.