But teachers do listen and they are intelligent enough in most cases to realize when they get upset parents/complaints they need to change their policy. Do you think they get some kind of joy in getting complaints?
I am not sure in this instance I believe the Principal is warranted either as I don't feel like it needs to initially go beyond the parent/teacher discussion. If the teacher was resistant or insistent on what they were doing only then would I involve the Principal. If they were receptive, understanding and indicated they planned to change their policy then I think it is fine. Teachers are professional adults and deserve to be treated as such..part of that is having a discussion with them about your concerns and issues and only if you don't get resolution do you escalate.
Why would you think I think that, all because I'd go about my complaint in a way you wouldn't?
I think what the teacher did was wrong, I think the Principal should know what she did. Thats fine if you don't, its fine if the OP doesn't. If it happened to my kid, I'd inform the Principal for the reasons I have already stated. And, of course those reasons don't mean that I don't think a teacher can figure things out on their own, or I think they like to get complaints, it just means I think the Principal of the school should know what the teacher's in their school are doing to someone's personal property.
Maybe in your school you only go to the Principal when you want to escalate something, but here our Principal is very involved with parents and with what goes on in her school, from little things to much bigger. I happen to like it that way, YMMV.